Penn Plax's Floating Thermometer works pretty well. It's easy to read and floats so it's at the top of your tank. My only compaint, however, is that the suction cups don't always hold then you have to reach it, grab it, read it and put it back.

Pros: Works well.

Cons: suction cup doesn't always hold.

works okay but the suction cup is a joke. we ended up letting it just float around the tank. our big angel fish had fun pushing it around.

Pros: reads temp

Cons: none

This is an inexpensive thermometer and is acurate, however I did have problems with the suction cups keeping in place.

Pros: Inexpensive

Cons: Suction Cups

I've used these for the past 8 years and find them useful. When we have pregnant fish I like to have the
water slightly warmer so this helps me regulate that temperature. The only thing negative is the fact that the themomentor often pops off the glass and floats
freely.

Pros: fairly accurate

Cons: pops off side of tank

Its so easy to put where you want it, and it does give a fairly accurate reading.

Pros: easy to use, convenient

Cons: none