Development
Jin is open source software, licensed under the GPL. You can download the source code and add any improvements. If I like them, I will add them to Jin. You can find a list of requested features and known bugs at the Jin Project Page. It's a good idea to contact me before you start writing any actual code - I may already be working on the same thing!
Test Jin Early
If you're a power computer user, but Java (or messing with other people's code) isn't your thing, you can test Jin by always running the most current code from CVS and reporting any issues you find. This will help by detecting bugs and other problems sooner than they would be otherwise discovered.
Artwork
If you have a knack for art and visual design, you can contribute piece sets, board patterns, icons etc. See the page on creating custom Jin artwork for more information.
Translations
If you know a certain non-English language very well (merely it being your native language is often not enough), you can help translate Jin. See the page on Localizing Jin for more information.
Send Money
The most straightforward way to contribute is to send me money via paypal. The money will go entirely towards Jin development. Specifically, it will most likely be spent on the hosting of this website, but I may also spend it on things like development tools.