Farms comment spam pack
Three plugins, edited to work happily with your WPMU install, are included in this pack - just drop it into mu-plugins and say goodbye to comment (& trackback) spam. The plugins included are Simple Trackback Validation, the Simple Spam Filter (don't post more than two http links in a comment or you get told off) and last, but certainly not least, Peter's Custom Anti Spam Image Plugin - a CAPTCHA solution that really works! (and all you have to do is edit the file to replace it with your own fun words) Much, much easier than SK2... and perhaps even more effective.