I wanted to note that the glass thermometers you buy these days are all alcohol based, not mercury (so no contamination risk if one breaks).
I have a couple digital of the type toxickitten posted and one glass as backup. Generally I really like them, but I started getting readings I just didn't believe with one (not just off, but bouncing around). I ended up tracking it to the light fixture, so I like having an alcohol-based one to help confirm things if the readings seem strange.
Also, one thing to note is that the digitals may seem more precise, but that model is still rated as being accute to +/- 1.5 degrees. I've worked with glass thermometers rated significantly more accurate than that, so I wouldn't trust that the digital readings are necessarily more accurate, even if they report more digits. In practial terms 1.5 degrees is plenty accurate for fish and amphibians, and I find the digital models much more convenient with multiple tanks.