Most newer (~98+) calipers and brackets are interchangeable (meaning you can just get WRX brackets and rotors to upgrade an RS or Outback, for example). As I mentioned, the 2-pot Subaru and Brembo rear brakes use a different backing plate, and Kartboy makes adapter brackets for them. So, in conclusion, you can pretty much put any set of Subaru brakes on any other Subaru with very little hassle.
Before "upgrading" it is important to keep in mind that many combinations will create a less than optimal brake bias and increase stopping distances. Legacy777 made a great spreadsheet with a bias calculator that can be found on his website. Anyway here are most of the different brakes found on modern Subarus. The information came from a lot of places like the brembo catalog, cars101, and various threads/FAQs on nasioc and scoobymods.
242x16mm - Smallest Subaru front brakes, found on FWD non-abs Imprezas (93-96). The caliper brackets are similar to those for early 260mm brakes (just shorter) and they use the same pads and caliper. These things fit under 13" wheels.
93-96 Impreza 2WD.
260x22mm - This is the most common front rotor size before 2000. It was found on most Imprezas and Legacies and uses a single piston caliper and bracket. Somewhere between 1996 and 2000 the caliper design changed depending on model, so while the rotors and piston area are the same, old and new calipers do not interchange between brackets. These fit under 14" wheels.
90-99 Legacy non-turbo/-GT/-Outback
93-01 Impreza AWD (non RS)
276x24mm - This rotor uses a two piston sliding front caliper and was first put on the Legacy turbo. There are three caliper/bracket designs. One for the SS/TW, an early Legacy GT/Impreza RS bracket, and a later RS/WRX/GT bracket. Post ~97ish calipers are interchangeable between brackets, and will also exchange onto the bigger wrx brackets. I have also heard that you can even put the later 1-pot calipers on these brackets. They fit under 15" wheels.
91-94 Legacy turbo
96-02 Legacy GT/Outback
00-04 Legacy non-GT
98-01 Impreza RS
02-06 Impreza non-WRX/-STi
294x24mm- This is the WRX front rotor. It uses a two piston sliding front caliper or the Subaru fixed 4-pot. There are two sliding caliper bracket designs: One is the same as the 98-01RS, and the other started use sometime in 02/03 (varies depending on model). Calipers interchange between these two brackets. 16" wheels are usually required. The 4-pots do not fit under 16x6.5 Subaru wheels but will work with 16x7 RS wheels (and possibly some forester/outback 16s). Both of these will also fit under many 15" rally wheels.
02-04 Legacy GT, GT LTD
02+ Impreza WRX
316x30mm- This uses a two piston sliding caliper similar to the other 2-pots, although brackets and calipers aren't compatible between other 2-pots. 17s are required with them. Applications:
05+ Legacy GT
Tribeca (5x114.3 bolt pattern)
326x30mm. STi rotor that goes with the 4-piston Brembo calipers. There are two rotors, the 04 has a 5x100mm hub bolt pattern, 05+ uses 5x114.3, although caliper mounting remains the same so you could use 05+ calipers on a 5x100 car with 04 rotors. There were some changes to the caliper over the years that involved some sort of stiffening, so from what I can tell an 07 caliper would be slightly better than an 04.
266x10mm solid- this is the most common Subaru rear rotor. It uses a single piston sliding caliper, of which there are a few variations. 90-95 Legacy sedans have a smaller piston than wagons and turbos, and there are a few bracket/caliper revisions.
90-99 Legacies/Outbacks with rear discs non-turbo
93-03 Imprezas with rear discs
04+ non-Sti Imprezas
06+ non WRX or STi imprezas
Foresters with rear discs
I think this is on non-turbo 05+ Legacies, and 05+ Outbacks as well.
290x10mm solid- This is well known as the H6 rear rotor. The caliper is the same as the ~99-04 calipers so many people buy these rotors and the taller brackets for a cheap upgrade.
00-04 Legacy/Outback non-brighton
266x18mm vented- This is the Legacy turbo rear rotor. The caliper is similar to the other calipers but is wider to accommodate the vented rotors. It was also found on the ver1 WRX (93-94).
91-94 Legacy turbo
290x18mm vented. This is a new rotor size for Subaru (in the US anyway). It uses either a 2-piston opposed caliper or a 1-pot slider. The 2-pot will not bolt up to other subarus without somthing like the Kartboy brackets or having a new backing plate pressed onto the spindle. There are two versions of this rotor in JDM land- one is for cars with an R180 rear diff which has bigger parking drums.
05+ Legacy GT (1-pot)
06-07 WRX (2-pot)
2008 WRX (1-pot)
B9 Tribeca (1-pot 5x114.3)
316x20mm vented. STi brembos. Also requires some sort of an adapter bracket or a whole new rear end since the STi has an R180. Once again there are two rotors for the different STi hub bolt patterns.
Some cars have 9" rear drums.
Some of them are: Legacy and Impreza Brighton models, Some 95-99 Legacy L models, some foresters and I think 02+ Impreza TS and OBS. (I'm not sure exactly)
To swap out drums, at the least you will need new parking brakes, backing plates, discs, calipers, and brake lines. It is usually best to get a whole knuckle/hub/parking brake assembly out of a car with rear discs. If you want to put rear discs on your front wheel drive Subaru, you might be better off getting parts from a 90-94 Legacy, since even the FWD versions had rear discs. Parking brake cables, to my knowledge, are not necessary. However the cables for a drum may be longer than discs.
Installing rear Subaru 2-pots or Brembos on a non 2-pot car:
The Subaru 2-pots will bolt to an STi Brembo backing plate and there are 290x18mm rotors with a bigger 190mm parking brake hat. This was primarily so rally cars can easily switch to 4/2-pot Subaru brakes for group N compliance and rally wheel clearance. In turn Brembos will bolt to an 06 wrx/Subaru 2-pot backing plate, but need a rotor with a 170mm parking brake hat (which is available from DBA). Here's a thread about fitting the brembos to other cars with the new Kartboy brackets and DBA rotors. They are for sale from places like fastwrx.com. There are also taller parking brake shoes available in Europe but they are expensive.
Some D-plate numbers
The Friction Materials Standards Institute (FMSI) is an association that assigns numbers to different pad shapes that are used as a guideline for pad manufacturers to refer to, and bring a common numbering system to the brake pad aftermarket. Virtually every pad manufacturer will either use the FMSI number in their numbering system or have a readily available interchange to cross-reference their part number to the FMSI number.
I should note that the caliper bracket, and not the caliper, is what determines pad shape. So while there are a few different pads and brackets, in some cases the calipers are exactly the same. For example, an 05 WRX caliper will bolt to a 99 RS bracket, even though the brackets and pads are different.
1990-1995 Legacy: D470
1996-1999 Legacy: D722
1993-1996 Impreza: D470
1997-2001 Impreza: D722
1991-1994 Legacy Turbo: DB1219?
1996-2002 Legacy: D721
2002-2006 Legacy: D929
1998-01 Impreza RS: D 721
2002-2005 Impreza RS: D 929
1/2002-7/2002 WRX: D 721
8/2002-2005 WRX:: D 929
1990-1999 Legacy: D1186
2000-2004 Legacy: D1379 or D770 (I think they're the same)
1993-1998 Impreza: D1186
1999-01 Impreza: D1379/ D770
2002 Impreza w/ rear discs: D1379/ D770
1/2002-10/2002 WRX: D1379/ D770
11/2002-2005 WRX: D1004
2003-2007 Impreza: D1004
2006-2007 WRX (and previous Black 4/2-pots)
Front: D 1001
Rear: D 961
I should probably put all the car/rotor/d-plate combinations in a spreadsheet or something. Outback should be the same as Legacy. I'll look into Forester and Baja stuff later I guess. There are a couple of things that I'm not clear on, like exatly when caliper/bracket designs changed between cars or which 00-04 Legacies got the 294mm front rotors. I might also be wrong about some of the forester front brakes. Also I've been meaning to add brembo or oem part numbers for the rotors or something like that.