It seems that every manufacturer has their own brand of couplers.
To name a few:
Walthers ProtoMAX (all metal)
Walthers Proto 2000 (plastic, discontinued)
McHenry (now owned by Athearn, plastic)
Bachmann EZ Mate (plastic)
Accurail AccuMate (plastic)
Sergent Engineering (all metal)
and the old standard, the horn hook or X2F coupler.
All couplers will work with each other with the exception of the Sergents and the horn hooks. The Sergents will work with Kadee scale couplers or McHenry scale couplers, but you'd have to slip one knuckle over another to get them to hook up. The horn hooks can be hooked up to Kadees if you jam the knuckle into the hook part (not pretty, but will work in a pinch)
Kadee is the de facto standard. There's the good old #5, or for those who want a scale (sized, not appearance) coupler, there's the #58. Kadee makes a variety of shank lengths and offsets as well as coupler heads. The Walthers ProtoMAX is a clone of the Kadee #5 with a slightly bigger knuckle. Rapido Trains (no relation to the N scale Rapido coupler) McDonald-Cartier couplers are an all-metal clone of the Kadee #5. Broadway Limited also uses a metal clone of the Kadee #5, but uses an interesting shank length.
If you think that the trackwork can use a bit of work or has dips or abrupt vertical changes, go with the Kadee standard head couplers. If you think that the track work is flawless and transitions smoothly between grades, then you can go with the scale head couplers.
When mounting couplers, it is a very good idea to body mount them as opposed to truck mounting them as truck mounted couplers can cause derailments if they hit anything or if the train is backed up too fast over a switch or if the curve is tight.
Eric from Boston, MA. Modeling Norfolk Southern and Pan Am Railways.