If anyone is using Bitcoin, the distributed digital currency, you can now run the Bitcoin server on your N900, thanks to a port by user doublec at the Bitcoin forum.

You can read about it and download the executable from here:
http://www.bitcoin.org/smf/index.php?topic=2125

This is the non-gui version of Bitcoin (the GUI version uses the wxWidgets library and will be harder to port). Here's how it looks: