How do bitcoin nodes initially find peers?
Although the torrent protocol requires a central server, you don't necessarily need a central server for a p2p network. The client only needs the IP address of at least one active participant. Connecting to this will give you other IP addresses to connect to.

So the only problem for a p2p network without a central server is: where to find the first IP address to connect to. Bitcoin solves this using a so called IRC bootstrap after the first installation. Basically it tries to find such ip-addresses on designated IRC channels.

This is only necessary on first run; subsequence runs it will just try ip addresses that have worked before.

More info on the IRC bootstrap: http://bitcoinmedia.com/the-irc-bootstrap-method-is-flawed/

EDIT: On further inspection, it seems that IRC bootstrapping is no longer used. See the link for details.

EDIT: See also this question regarding the initial connection (bootstrapping)