The largest bitcoin exchange is MtGox: the MtGox "last" value is usually what people refer to when they say "a bitcoin is worth x US Dollars". Go to https://mtgox.com/ and look at the very top of the page for this value along with a few other trading statistics.

You can buy/sell bitcoins at different rates via other exchanges, but you'll probably get the best deal on MtGox. One potential downside is that MtGox requires you to verify your account in order to trade- this requires a government-issued ID and proof of address such as a utility bill. Other sites will sell/buy bitcoins to/from you without this information but may charge you more/give you less than you could trade for at MtGox.