Check out Soulseek, it is very similar to how I remember the original Napster. Lots of obscure rarities on there music-wise, I've not tried it for anything else.

The one P2P thing I am nostalgic for is Audio Galaxy - it had an awesome system where it they indexed everyone's content on their website, so you could see every file that had ever been on the network and add it to your "want list", then when that person came online, it would start downloading. To be fair "wish list searches" in Soulseek serve a similar purpose, but I loved that ability to browse every file that had ever been online!