My 3 favorite exchanges (I've used about 10) are:
1) mtgox - has the lion share of liquidity, leads the price movement, opportunities. Accepts dwolla deposits within 24 hours. Mtgox is managed well, though seems to be the target of some ddos activity. It can also lag a little under heavier trading. Commissions start at 0.6% but anticipate delays relating to paperwork (submitting ID, proof of residency) if you will be doing bank or dwolla transfers.
2) bitfloor - hands down the overall best managed site. The price often lags mtgox, which means you can often buy at quite a lower price when gox is rising. Cash deposits into Chase or Wells Fargo banks are credited within a few hours - definitely the fastest and very safe. Did I mention liquidity rebate for market makers? Rather than pay commission you can be earning it (0.1%) by placing orders on the book - or if you need your order to go through immediately and buy something already on the book commission is 0.4%.
https://bitfloor.com/3) bitme - another nicely designed site. Aside from having a low volume (I'm hoping more users begin using and this becomes a viable exchange), they have a very generous 0.55% market maker rebate; market takers pay 0.6% commission. They take direct deposit (ACH) payments from banks.
https://bitme.com/honorable mention: campbx. takes dwolla. 0.55% commission. reasonable volume, some lag effect for good arbitrage similar to bitfloor above.
https://campbx.com/faq.phpI'm also an advocate of the OTC marketplace on freenode IRC: #bitcoin-otc
It's an unregulated marketplace, so use caution when balancing risk and trust. There is a bot that manages reputation scores (the "Web Of Trust") but also rely on other chatters for feedback. ASK QUESTIONS AND BE CAREFUL. You can find people there to trade for dwolla, paypal (if you have a good reputation), or all manner of other things like gift cards, moneypak, etc. ASK QUESTIONS AND BE CAREFUL. There are risks for any/all exchange methods. There are many scammers ready to take advantage. ASK QUESTIONS AND BE CAREFUL.
http://bitcoin-otc.com/http://webchat.freenode.net/?channels=#bitcoin-otc