I'm the (by far) main developer of OpenSettlers (A very buggy branch release is here, ~1MB). Right now, I'm working to get a specialized game engine ready solely to implement all the different Catan variants. The standard variant kinda works, but with some bugs in the game engine and render engine. The project is now about 8 months old, and the coming 8-12 months it will likely reach version 1.0. It's open source (GPL), and uses GWT to have the game 100% natively html5 playable in the browser.

There's a side project which I did mostly the graphics for, OpenSettlersGraphics, of which you can find my super awesome gimp skills here. It's mainly created so I have feedback when debugging te game, so I know what's going on. Most icons are famfamfam icons I reused and edited. 

It would be supercool if someone with more serious graphics skills would commit (even very small) contributions. My Gimp skills go only so far, and I think at this time it's more important to make te game less buggier and more playable. so I'll be spending my free time on that. I updated the graphics todo list on the wiki