Admittedly, I got off on a tangent there.

OK:

1) Verify e-mail addresses. Do not allow duplicate e-mail or IP registrations.
2) Require PCRE regular expression usernames.
3) Utilize the Question And Answer registration feature. Use questions and answers only someone with some level of knowledge about your forum specifically would know without having to look it up. Like who or what or how many of something or what colors appear in your site banner. Bots can't answer that.
4) For new users, make any post with more than 0 URL links go into moderation. No user should be linking to anything in their first post. You can also create another usergroup and promote new users to that group after x number of posts if you want to give users the ability to post links after enough posts to know they are not a spammer.

All of the above can be done without installing a single modification.

Many noobies first posts are youtube music videos, perfectly legitimate stuff and it really does irritate new users to get that notice that their posts are moderated. Same way it really irritates people for their account to be moderated, has to be approved by the admins before they can post. I don't recommend either of these, personally.

from personal experience, agree 100% and for a forum with no or limited staff this can be exhausting. All the other recommendations are good ones though :)

I still stand behind Spam O Matic though ;) (sorry guys, it saved my forum got to rep it lol)

My personal preference is that users post a "Hello" thread as their first post.

But to each forum it's own.