As you have no doubt discovered there are a lot of them out there. If you want to use one, I think it’s a lot about taste and personal preference (both your and your users).
The only place I’ve seen DreamHost use a captcha that immediately comes to mind is in their custom panel for when you have more than “N” number of domains/email accounts/users etc., but I have no idea what they are using. I think there is also one for “new” wiki account additions on the DreamHost wiki.
I happen to like reCAPTCHA myself, and I kinda like that it may actually being useful for something else as well as thwarting spamtards.