I don't know about "cool", but we changed several Auto-Number prefixes so the records resulting from each WS are more-easily distinguishable.
For example, the Customer Number now begins with "CUST", the DRF with "DRF", and the Project with "PM".
Any PLM work is cool! Actually, way back when we posted about the importance of autonumbers. Not only does it help the end user when looking at records, but it's great for importing.
I'm working on creating an example using derived fields - it is Friday after all! Means another Feature Friday post will be heading your way on the Under the Hood blog (figured if I posted about it here, I'd *have* to make sure I publish it before EOD!)