Hello,
I couldn't be more happy to hear from people who want to start contributing to MooTools. I suggest getting in touch with us on IRC (freenode #mootools, ping someone like me (cpojer), Timmeh, arian, fakedarren or just stick around for a day :) ). IRC is still the best way to start contributing - actually getting in touch with the developers and communicating with us is really the best way. Open Source is about communication. Feel free to add me on Google Talk and I'm sure we'll find something you can do, if you really want to =)
