The best option for US users, Coinbase lets you buy and sell bitcoins using either a US bank account or credit card. Approval is fast. The price is based on a rolling average of all exchanges.
Buy bitcoins using your mobile account. Works with every mobile phone or landline. No registration required. No credit card required. Can only be used three times a month, but has no maximum purchase limit.
BTC-e is one of the biggest bitcoin exchanges. It’s advantageous if you want to also trade litecoins and a few other altcoins.
The most well-known bitcoin exchange. Sign up for an account, verify yourself and you can purchase a large number of bitcoins.
Bitstamp.net is one of the biggest bitcoin exchanges. Buy and sell bitcoins with other users. Identity verification is required to deposit funds.