Depends on where you are, I guess.
I am from Germany, but registered on Mt.Gox. Though I haven´t traded there for a while now (last time back when btc where at about 100$), but the Transfers to and from my Bank Account usually took only 2-3 days, which I considered okay for overseas trading. There are German Exchanges and Marketplaces which are even faster, I guess the same is true for you, wherever you are. If you are in the US, I would think mt.gox is still a viable option.
For other trading, namely trading Alternative Coins or Litecoins versus the Bitcoin, you could consider Crytpsy.com. They have their teething problems still, but they are getting there. I like it very much, since you trade one virtual currency against the next. If you feel you are done, you cash out in Bitcoin form to Mt.Gox, for example, and turn it into "real" money.
Those Alt-Coin Exchanges are really a nice place to play around, though I wouldn´t put anything in it you can´t afford to loose. But you can start small and try to trade up. Alt-coins are a bit like stocks. If you follow their respective threads on here, read up on them a bit, see how the acceptance and market share is, you can form predictions, also by watching the charts. Then you can buy when a specific coin is in a slump, for whatever reason (for example, some coins get mined a lot on some days, so many newly minted ones flood the market, price goes down, while the difficulty to mine them rises. When the miners move on, usually the coin price rises, then you can sell). Mining is just one factor though, many rise and fall with Bitcoin value or other factors. The fun is to find out, which factors
I mine a bit myself and put mined earnings on Cryptsy, then sell or trade with them. You won´t get rich that way, but it´s a nice side income.
