How Do I Prevent Junk Account Creation?

I want to allow readers of my WordPress-driven sites who comment frequently the ability to sign up and have their own account on the site. However the ratio of actual readers versus spam scripts using the sign-up form is something like 1:100.

Is there a reasonable solution for preventing these bogus accounts from being created while still allowing reader sign-ups? Last I looked, I couldn’t find anything for account creation like what spam-prevention measures are available for comments.

Sabre is great plugin, it allows me to block dozens of spam user registration per week.

It has also many useful features to control the style and captcha’s complexity.