Pay Per Click Keyword Discovery
Keyword research is the "meat and potatos" of the PPC world. You have to
find keywords that are relevent to your product, but are not so competitive
that you can't afford them. This requires a little creative muscle. There
are several tools out there that help in this process.
The number one greatest tool is the famous Wordtracker which pulls from various actually searches on the web and has such nice features as a related keyword function, a popularity database, and misspellings. They offer a free trial so you can judge for yourself.
Google Adwords offers their keyword suggestions tool. This is an excellent first stop. Of particular note is their Similar Keywords section. This gives you a hint on what Google considers related to a main search, and is useful for using the "broad" match feature in AdWords.
Following this are two excellent tools from Overture - the search term suggestion tool and the current bid tool - both of which are essential tools in a webmaster's toolbox.
All these keyword tools can be addicting, and there are many out there. An excellent page to find more good links is from the Open Directory Project. There is enough on that one page alone to keep anyone busy for several days.