Best keyword research tools?

[Blarian]

I'm on a search for the best keyword research tools (keywords will be used for PPC.) I'm looking to enter 10-15 generic keywords and get a big list of related keywords. What's the best tool to use for bulk keyword research? (In general, entering 10-15 generics I want to get atleast 1,000 keywords even if they are only remotely related, then I will be able to use them on PPC and eliminate the ones that don't perform myself.)

The only ones I've heard of are wordtracker and rapid keyword, anyone know any better tools? (paid or un-paid doesn't matter, just looking for quality.)