l33t! "translates" from English to l33tspeak, and let you choose between different l33tflavors, from "basic l33t" all the way up to "hard l33t".
You can define your own rules or tweak the hardcoded ones by modifying the l33trules.xml file (rather self-explanatory).
You'll need the .NET framework 2 (available on microsoft.com) to run l33t!
Download
l33t!
Further informations
L33t page on Wikipedia