I went ahead and created a modified Bitcoin client with a brand-new block chain and a few other changes (detailed below) that will make it independent of the "production" Bitcoin network.
I intend to use it to test out some code I'm writing for my next project; I don't want to accidentally break (or slowdown or otherwise interfere with) the real Bitcoin network. It should also be useful for anybody who wants to see if they can purposely break Bitcoin (for the purposes of a security audit).
Code is hosted at github: http://github.com/gavinandresen/bitcoin-git
Patches to go from 'production' bitcoin to 'TEST network' bitcoin are also at github: http://gist.github.com/502460
... so go nuts and start hacking.
Changes are:
+ Listen port is 18333 (instead of 8333). Uses 18332 for JSON-RPC (instead of 8332).
+ Uses irc.lfnet.org channel #bitcoinTEST for bootstrapping.
+ Never tries to connect to "well known" nodes for bootstrapping.
+ Uses bitcoinTEST for the default data directory (e.g. ~/.bitcoinTEST on Linux, %APPDATA%/BitcoinTEST on Windows, etc)
+ Uses a different ADDRESSVERSION for Bitcoin Addresses (0xff instead of 0x0), so you can't screw up copying and pasting Bitcoin addresses (if you try to use a TEST address on the production system it will tell you it's invalid; you can, however, lose TESTcoins by trying to send them to a valid production address)
++ Has a brand-new block chain (with a different genesis block)
++ Initial difficulty is 4 times easier than production Bitcoin
++ protocol message header bytes are { 0xfa, 0xbf, 0xb5, 0xda } instead of { 0xf9, 0xbe, 0xb4, 0xd9 }
(16 July: "++" stuff added in response to the suggestions below in this thread)
A very generous TEST Bitcoin Faucet is running at: https://freebitcoins.appspot.com/TEST/
I think the TEST and production GUIs don't play well with each other; at least on my Mac, I have to quit the "real" bitcoin before running TEST. That's probably an easy fix, I just need to figure out how...