Coinbase.com is by far the easiest to simply buy or use Bitcoin if you have a US bank account (and it is a YC company). Unlike MtGox.com (originally the most popular) the site is fast, stable, well designed, and can connect directly with your US bank account. They've also got a set of merchant tools and APIs.
The only downside is that it's not a real time trading platform / you wouldn't want to do day trading of BTC on it.
This link will give us both $5 in BTC if you buy 1 BTC.
https://coinbase.com/?r=4fedfb2d6cbd3b0003000845&utm_campaig...