I use the Hydor 7.5W mini heater. It works great for small tanks. It isn't programmable, but keeps the water about eight or so degrees warmer than room temperature. (A constant 78-79 F in my condo.)

 

I have two 2.5g Aqueon Mini-Bows, and I use a Hydor Theo 25w submersible heater in each tank. They look a bit big but since they're mostly clear they don't look that obtrusive in the tank. Most importantly, they're adjustable, and they are fairly accurate. I've had a couple of random bouts of cold here in Texas and I can count on waking up to see that the tank is still 78F, even if my room temperature is 60F.

 

+ 1 for Hydor Theos. I use a 50 watt in my 2.5 gallon and it works fine. You just have to watch the temperature the first day so you can adjust the temp accordingly. Usually you have to set it at a lower temp if it is a higher wattage than recommended.

 

I have a 25 watt Marineland stealth visitherm in one of my 2.5 gallon critter keepers.