Actually, the best I've found is the adblocking CSS file that ships with Camino nightlies:
http://www.caminobrowser.org/download/releases/nightly/
Download, open, then Show Package Contents on Camino.app and drill down to Contents -> Resources -> ad_blocking.css. Drag it out, put it where you'd like and set it as your default CSS file for all sites in OmniWeb - it'll make an enormous difference right off the bat. It's updated once a month. You can keep an eye on new commits of the file here:
http://bonsai.mozilla.org/cvsquery.c...oot=%2Fcvsroot
HTH --