What you'll want to look for in a good heater is something adjustable with a thermostat and fully submersible. for a 2.5 gallon, something about 25 watts should do the trick. Hydor, Elite, Aqueon, and Petcos brand are a few good brands, though just about anything with everything I mentioned previously should work about the same. I'd advise staying away from Top Fin heaters though....I had a bad experience where one contaminated my tank once.
Also, this is the heater I use in my smaller tanks. Only problem is shipping takes a couple of weeks and the cord is a little short(but that can be fixed with a small extension cord).....but otherwise it works very well.