I use BTC-e, it is a good service although their prices don't correlate too well with other services.

There's https://vircurex.com/ which I'm beginning to use as my backup exchange (and I recommend every trader to have atleast one backup exchange set up, because when you would need to trade the most, you usually have problems doing so).

Also many services like cryptoswap.com are used but personally I've no experience nor I've heard any reviews regarding them. Needs investigation.

I have never used https://mtgox.com/, and hopefully never will, it being most famous but also probably the most inconvenient service available.
https://www.bitstamp.net/ is a well-working service with 1-3 days business delivery within SEPA for bank transfers and a wide selection of other payment methods.

Too few multi-currency exchanges currently in my opinion; we DEMAND more of them. With altcoin support.