i see many have voted for faucet, but as long as faucet payment is very tiny such as being 100 to 300 satoshi it really isn't something we can play with.
i saw a suggestion somewhere which i think can apply here and that was to give play money, for example you give 1BTC non-withdraw-able and let players play with that to test things and have fun.
Very true, however most users tend to want "real" bitcoins they can bet with.
OP, are you planning to use the moneypot API? If so, they have a built in faucet that you can integrate into your site.
I think chat rains would be a very nice feature, however perhaps implement that later when you have an active, chatting, userbase.