How about an online spell check tool? Maybe you can even allow users to input bbcode as I often see people making unneeded typos in their threads. This tool will save the embarassment of starting a threads titled "Proffesional Poster!" for example.

A basic database could work or you could use an existing one.

The 1913 Webster's dictionary database was made open source several years ago and can be found here:

ftp://ftp.dict.org/pub/dict/pre/