Arial is my "safe" font that I use all the time.
But I've been using Trebuchet a lot lately. It's common enough that practically everybody is going to have it. It's used all over the place at StackOverflow, FeedBurner, and a lot of other popular sites these days.
But, just in case the user doesn't have it, I'd use CSS like this (just like StackOverflow has it)...
font-family:Trebuchet MS,Helvetica,sans-serif;
