Security: Block bad spiders and bots from access to website using htaccess and HTTP_USER_AGENT

When building an htaccess rule to block common spiders and bots, what HTTP_USER_AGENT headers should be filtered?

Looking for a Java User Agent String Parser

Does anyone know of a great library in java for parsing user agent strings? We have written a custom one, but this seems like a common problem for many people. I would guess there is a good library available somewhere.

Detect 64-bit or 32-bit Windows from User Agent or Javascript?

I want to offer the right version of a download. The versions I have are: