I had to research into captcha solutions for a asp.net project and narrowed it down to the following 2 which I thought I’d share with you:

1. Lanap BotDetect – http://www.lanapsoft.com/products.html – currently $59 for 1 server/website.
Pros: Web control. Easy to Use. Very flexible configuration.
Cons: You need to host it which is a negligible issue practically.

2. reCaptcha – http://recaptcha.net – FREE

This is a CMU (the inventors of Captcha) project and uses Captcha to digitize books. Novel idea.
Pros: No hosting overhead, which also could be a downside if your application is accountable for downtimes.
Cons: Hosted externally so not within the business’ control

I would go with #1 if you want to deploy Captcha in an Enterprise-level application and #2 if you don’t want to pay a dime and don’t really care about downtime(s).