Here are a few ideas:
* Improve/extend the shoutcast server in Joe's book "Programming Erlang" (1st edition; I haven't read the 2nd edition)
* Make a simple dropbox tool to sync folders across machines
* Make a simple map/reduce tool to learn about that paradigm
* Build a chat server with web interface (this one is fairly common)
* Interested in VOIP? Try making something that can allow you to make VOIP calls between computers in your house
* Build a web server (also common enough to find lots of examples and tutorials)
* Twitter clone
* Log aggregator
* BitTorrent client
* Distributed hash table or NoSQL database
* Something to stream data (logs, newsfeeds, analytics) to or from multiple clients
* ....
I hope this helps :)
[ Edit: formatting is really hard on HN... :( ]