for hand-registrations, i like the artificial intelligence at fabulous.com when they suggest similar names to your search. for aftermarket- i like the boards. for expired domains- http://pddw.com by IPalert, Pending Delete Domain Watcher
i like to run the niche key words in a key word tool. then see if those key words are available or variants for those key words. over 95% of them will not be available for competitive niches but you just gotta be creative with the word combinations.
go to name.com ==> enter your domain, check all extensions name. and click check, then you can see if your domain is available, and they give many suggestion.