Plugins are written in C language, you should take a look at the tuxbox community website where they host the source code to tuxmail, lcdmenu and loads of other stuff. You should also take a look at the CDK.
Source code can be found here: [DLMURL]http://cvs.tuxbox.org/cgi-bin/viewcvs.cgi/tuxbox/apps/tuxbox[/DLMURL] or alternatively through the tuxbox cvs service.
I think the CDK is here:
[DLMURL]http://cvs.tuxbox.org/cgi-bin/viewcvs.cgi/tuxbox/cdk[/DLMURL]
and the random plugins can be found by just crawling the web (i.e google)
Liam.