Here's a cute way to generate Bitcoin key pairs using only bash and some fairly standard command line utilities (disclaimer: I wouldn't use it to generate production keys).

https://gist.github.com/colindean/5239812

This was actually put together at the Bitcoin dawn of time by a bitcointalk user called grondilu.

Here's the original (circa 2010) thread:

https://bitcointalk.org/index.php?topic=2461.0;all