Smack is an easy to use Java API for XMPP (as in Jabber).  If you haven't heard of it, check it out.  I just wish it was as easy to use other IM technologies.

I felt into a mental hole today and found myself at Petri Nets World.  Since I never had formal training in CS, I haven't had the need to discover Mr. Petri's works until well into my easy-as-pie-compared-to-physics career.  To be truthful, I didn't understand what all the hoopla was about.  Anyway, the website is a good source of information and, particularly, links to useful tools though.  BTW, I like PNK (guide) for testing Petri-Nets.