Right now I have:
160 ? + acrylic 3 teir display with 2 - 250 watt titan.. won bros
(the display loses alot of heat since it has 3 open levels)
1- 120 gal. with 2 - visa therm 200 watt submersiables
1 - 90 gal. with 1 250 watt titan.. won bros
1 - 70 gal. now with a 250 watt titan.. won bros
6 - 55 gal. with 200 watt whisper submersibles
2 - 55 gal with whisper hang on 200 watt
2 - 55 gal with whisper hang on 150 watt
3 - 15 gal with ebo 75 watt submersiable
1 - 15 gal with a visa therm 100 watt submersiable
6 - 15 gal with pennplax - pc therm 100 or 150 watt hang ons
6 assortes size from 25-48 gal with pennplax 150-200 watt hang ons
I think thats everything thats currently running.
Heater size depends on room temps and such though,so other people might have large(er) heaters in small tanks,I have a heater in my basement so that cuts down on the size of heaters I need
The penn plax aren't great and are sort of junk,but at 20% of what some other heaters cost ,they work

.
Some of whisper and ebo's are about 8-10 years old and still work great so I guess you get what you pay for