Domainr is a pretty awesome tool by a few guys I met this afternoon. Basically you put in your domain idea, and it shows you all sorts of variations based on different domain name endings (.com and .net along with all the foreign endings). In addition it will show you variations that are close to what you have (which is super helpful since so many .coms are taken).
As a side note (mostly because I’m not sure whether I’ve ever written it here), here is my philosophy on buying domains (aka how I justify spending $1000 a year on them): Whenever I have an idea that I think is even kind of good I buy a domain name for it. This a) names it, giving it some legitimacy and b) gives me a tiny kick in the ass (while $10 isn’t a lot of money, it’s enough to at least push me to get started). Then I just need to build one thing a year that is worth $1000 to justify the expense (of course how I calculate that value is completely made up — but I figure I built about four sites worth that this year).