BustAName

This app is hands down the greatest tool I use when looking for a new domain.  It has so many amazing features, but just listing them off here doesn't do it justice.  You really should go try it out for yourself, it's free and you don't have to signup for an account.

BustAName has two great feature, the first being that you can just type a domain in and it will tell if you if the .com, .net or .org are available.  This is extremely useful when your brainstorming or trying out variations of  word.  The other great features is it's ability to combine words in a list into into two word domain combinations.   Each word in the list has it's own thesaurus for finding synonyms which comes in really handy for finding even more variations that you didn't wouldn't have come up with on your own.   This is really just the tip of the iceberg with this app, you should absolutely  go check it out.

Domain Pigeon

This site is sort of hit or miss, but I like the concept of it.  It takes a lot of 5 letter or less gibberish domains and lists them all in one place.  Most the time, the domains listed are crap, but every now and then you see something that is actually good.  A diamond in the rough.

Domainr

This is a clever little tool to help you find domains like: del.icio.us.  These domains can get out of hand fast, so you have to keep it simple and creative when choosing one.