I've started to have a real problem with comment spam. For a long time I thought I was going to get a pass because I use a home-grown blog system, not MT or WordPress. I was wrong!
In the last 6 months I have over 2000 comment spams. Until now my only protection has been a general ability to turn comments off on a per-post basis and single IP address banning. That's not going to be effective in the long term.
For now, I found lots of lists of other peoples IP ban lists and have included 7000+ addresses from Gloved.com.
The next step will either be moderation – which could become a torrent if the spammers hit and would need a way to deny a large list in a single step – or some hurdles to posting like a simple math problem. I hate the image-based validations and eliminating those too dumb to do arithmetic might be a nice bonus.