Apache / Mod Security
If you're using Apache then also check out "Mod Security". This can help block many types of http attack and prevent information leaks if someone does manage to break in.
"DOS Evasive" is also a good one to try, but it seems to have gone AWOL from (what was) its offical web site so you will need to do some digging to find it. Alternatively, you could configure your firewall (pf, if you're using OpenBSD - and why wouldn't you?) to help mitigate DoS attacks.