Other options in case you are not too comfortable with embedding a Google service into your forums:
Honeypot captcha hidden from users via CSS:
http://haacked.com/archive/2007/09/11/h ... tcha.aspx/
ASP.NET based captcha:
https://stackoverflow.com/questions/847 ... approaches
https://www.codeproject.com/kb/custom-c ... ntrol.aspx
The author of that StackOverflow post also mentioned a few text based options:
ASCII text captcha: \/\/(_)\/\/
math puzzles: what is 7 minus 3 times 2?
trivia questions: what tastes better, a toad or a popsicle?