Rails server says port already used, how to kill that process?

I'm on a mac, doing:

rails server

I get:

2010-12-17 12:35:15] INFO  WEBrick 1.3.1
 [2010-12-17 12:35:15] INFO  ruby 1.8.7 (2010-08-16) [i686-darwin10.4.0]
 [2010-12-17 12:35:15] WARN  TCPServer Error: Address already in use - bind(2)
 Exiting
 

I know I can start one on a new port, but I want to kill this process.

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