Today I learned about a new thing: Referrer spam.
The idea is that you get your referrer URL put into as many web sites' logs as possible, since those logs are sometimes visible to web spiders. The referers are usually presented as links, so, say, googlebot will pick them up and rank the referrer page as being linked from many sources.
Tricky, insidious, wasteful, and kinda clever. :-)
Solutions include modifying .htaccess and/or adding a rule to mod_security.
There's also the Awstats vulnerability, which someone with a North Korean IP is trying to sniff out on my server. Good thing I don't use Awstats.