In Australia I frequently use wotif.com. I don’t know if it is international or just here, but it is reliable and usually has good deals. Sometimes though after I see the price on wotif, I will call the hotel directly and ask them if they will beat the price, often you can get an even better deal that way.

I’ve used Priceline and I’ve liked it, but you can’t really specify the hotel that way. So sometimes you get a surprise, not always a nice one. I do agree with @rooeytoo that
calling the hotel directly – not the 800 number, but the reservations desk – will often get you
a nice deal because the person you talk to actually has the power to work something out with you. Tell the truth, ask for what you want, make an offer. If they can’t do it, they won’t, but if they can, it’s great.

Priceline has two options—one where you can find specific hotels at discounted rates, and one where you can “bid” on what you want to pay and they’ll find a hotel that matches your requirements. I’ve had great luck both ways—including getting a hotel in New York City that would normally be about $700 a night for just over $200.

Travelocity has given me good package deals on flight/hotel combinations, as has Orbitz.

Best thing to do is to open multiple tabs and compare rates against all the sites and find the best deal that way.

Sidestep.com always seems to be the best for me. The Jet blue website can also have some good deals at times. (I think that kayak.com is basically the same thing as sidestep.com)