Pixelgeek,
We're currently using the Stop Forum Spam extension. However, given we're a volunteer organisation, it's more efficient for us to use that in combination with a Captcha, than to require all new registrations to be vetted by an Admin.
The Captcha disappears after you make 10 posts. We dont' consider that a difficulty in terms of registration of new members. Yes, it's annoying for the first 10 posts, but this has proven to be a good balance between ease & effectiveness on many forums, not just here.
I hope you'll bear with this for the remaining posts needed to clear the Captcha.
Cheers,
Worktroll, Admistrator.