Fully Customizable Ship and ILikeCheating (Sen's creative-mode alternative) are the only two "major" mods we allow. Note I say allow, not support; if you can get by with a vanilla game, we prefer you do so. Beyond this, client side UI-type stuff is okay, although we recommend you run it by us first to be safe. Anything that adds custom assets (Creative Mode, racial mods, other custom ship mods, etc) is strictly prohibited.
Very big emphasis on the prohibition of custom asset mods, too. These mods tend to create all sorts of problems on the server as a whole. Generally increasing the frequency of stuff like lag, world wipes, general corruption, etc. Definitely stay VERY clear of mods like those. :3