I probably go to Domaintools.com more often then I check my email - and that says a lot. They have a number of nifty features especially if you opt for a paid membership.
They now have a free domain typo tool available at their site. Since I do some proactive copyright/tm searches for clients, I'm always on the lookout for more ways to research. I ran a couple of recent searches I did for clients and found a few domains that I hadn't come across through other methods. (Of course, I wouldn't register tm/copyright domains for my own purposes!) It also has a few other capabilities that make it a cut above other typo tools.
Check it out yourself before they start to make it a paid service.