OpenGroupware.org is mostly written in the Objective-C language.
<br />
Objective-C (or ObjC in short) is a simple, object oriented extension to 
ANSI-C and the compiler is part of any gcc compiler.
<br />
To learn more about Objective-C take a look at our 
<a href="http://www.opengroupware.org/en/devs/resources/">Developer section</a>
or for a short introduction, consider reading that 
<a href="http://www.linuxjournal.com/article.php?sid=6009&amp;mode=thread&amp;order=0" 
   target="ext">Linux Journal article</a>.