2-5gl 'tanks' are worlds apart from 20 long and the popular 12gl/17gl/25gl tanks.
My short answer for mini tanks: none that I can find. For larger tanks, check out Cobalt Aquatics Neo-Therm line.
With mini tanks, I ended up heater-less. If your room temperature does not drop below ~64F, you can keep a lot of "tropical" fish and plants that prefer / tolerate cooler water: guppies, Endlers, dwarf corries, and fish that needs higher oxygen content like stream loaches.
For mini tanks, a lot depends on your life style: I tend to work at night and to light my small tanks at night with spiral CFL bulbs - they produce enough heat to keep the tanks heated. Use more hardscape: lava rock and pebbles retain heat longer. Another consideration is tank placement: in sunlight, next to a desktop PC or other stable and known heat sources could also work, based on your risk tolerance.
Good luck.