BotSmasher stops spam by checking comments and registrations against the BotSmasher API.
We're working on coming up with tools that can be installed to provide filtering for other contact forms. But you can also contact your favorite contact form creator and invite them to integrate BotSmasher into their form as a filtering option!
Right now, yes. The standard API key grants you 100 spam checks a day. This may change, but if you need more queries, contact BotSmasher.
Catching every piece of spam is, honestly, pretty darned hard. I can't really guarantee that BotSmasher will catch every piece of spam you get -- but if you keep submitting missed spam to the database, it'll just keep getting better!