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.
Coinbase is nice, but dealing with their support is not a fun experience at all. For about a month they held about $2000 of my funds due to one of my purchases being marked as high-risk. Support took anywhere from three to five business days to respond to each email. I eventually got my money back after threatening with legal action. Would not do business with them again.
In case you are European, I can highly recommend bitcoin.de for hassle-free, legit buying and selling of Bitcoin. Similar to Coinbase, they're less a day-trading platform and more a place to buy and sell Bitcoin within a few days. They have a partnership with a "real" German bank (Fidor) which makes them highly unlikely to have regulation issues any time soon.
I have had no issues with them, was about 48 hours after signing up to get my first Bitcoins. Supports 24 hour bank transfers or 1 hour cash deposits if you need them fast.
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...