Ok, well firstly, most BC/BF/BJs have a rear swaybar. The exception to this is the 92-94 L models. All of them. I always thought the wagons did but apparantly not. According to the compendium, LS, LSi, and 90-91 L models have a 16mm rear bar. turbos and air suspension cars have 18mm. There are also two different setups for rear swaybars. The 90-92 cars have a droplink setup, which is a vertical endlink that fits into a hole in the rear lateral link.
Picture:

new stuff: Here is a thread that explains how the older setup works and what parts you can get from whiteline to upgrade your droplinks:
http://bbs.legacycentral.org/viewtopic.php?t=34005
Anyway, back to our regular scheduled program:
The 93-94s, along with most other newer Subarus, have C-links that connect to tabs on the endlink:

AFAIK, all the BC/BF mounting brackets are the same, as well as pretty much every other Subaru mounting bracket. Outback/forester brackets might be taller.
It should also be noted that the mounting brackets on 04+ Imprezas are beefed up, and are a good option if you don't want to spend all that money on the HD whiteline parts. The bushings for those brackets are also different. The whiteline part number for them is W0406-[mm] as compared to the stock W0404-[mm] d-bushings. Click for pictures.
So, anyway, if your car doesn't have a rear swaybar, it's not terribly difficult to install one. The benefits of an rsb include better handling due to more evenly balanced roll stiffnesses and less body roll. It really makes the car feel a whole lot better and creates less understeer.
What you'll need to do this:
-A swaybar*
-Rear lateral links with endlink mounting tabs
-Endlinks
-Swaybar mounting brackets and the right bushings for the bar you're using.
-Possibly a thread chaser
-Probably new hardware to mount the lateral links
-An alignment afterwards
*Only swaybars made for our cars will fit without issue. The tire well is in the way of impreza bars, although they can be made to fit by "moving" the bump out of the way. I'm thinking about trying to fit a 20mm wrx rear bar with creative use of a deep socket and jack.

If you shop around and buy the parts used, you should be able to get everything for around $100.
Once you have all this stuff, you'll need to replace the rear lateral links. This part may or may not be easy. I had a hard time with the bolt that connects the link to the crossmember. It's a cammed bolt that is used to adjust toe, and mine was fused to the metal part of the bushing. After a whole lot of prying and burning, I cut off the bolt with a sawzall. Before this I had brought it to a shop, and their impact gun couldn't get the bolt out of the bushing. The side that connects to the knuckle didn't come out very nicely, either, and I ended up destroying the bushing.
Burn baby burn:

Ha ha take that:

I had to bring the long bolt that connects the links to the kunckle to pepboys to get the remains of the bushing off. I recommend just buying new bolts, nuts, and washers beforehand, then it won't matter if you have to cut things off. I waited 3 weeks to get the cammed bolt because we kept ordering the wrong thing.
Alright now things are looking better:


Time to get the mounting brackets on. The holes are already in place in the frame, so all you need are the mounts and hardware. The right one is a little difficult because there isn't much room to get at the front bolt, which is above the gas filler tube. I found it was best to take off all the plastic shrouding and such on the tube to get at the hole, and also to chase the threads because the bolts didn't want to go in very easily. I don't remember the pitch off the top of my head.
Here you can see that there isn't much room:


Putting on the swaybar is pretty easy. You just need to feed it over the exhaust and bolt everything down. Endlink bolts should be torqued to spec with the car on the ground.
Since you messed with the cammed bolt that adjusts toe and put on different lateral links, you're going to want an alignment. You should probably mark where they were in the first place, so you can drive around a bit without trashing the tires.
Other threads of note discussing rear swaybars:
http://bbs.legacycentral.org/viewtopic.php?t=25767
http://bbs.legacycentral.org/viewtopic.php?t=22175
http://bbs.legacycentral.org/viewtopic.php?t=19565
http://bbs.legacycentral.org/viewtopic.php?t=15736
http://bbs.legacycentral.org/viewtopic.php?t=15967
http://bbs.legacycentral.org/viewtopic.php?t=2366
http://bbs.legacycentral.org/viewtopic.php?t=986
http://bbs.legacycentral.org/viewtopic.php?t=195
Another option could be to buy the endlinks that attach to the end of the lateral links. Poltec makes these, and you'd have the additional benefit of adjustable endlinks with spherical bearings. Price is $170.
Did I miss anything?
Edit: Oh yeah, I forgot the bonus picture:
