RustChance is one of the most popular RUST gambling sites. It has really active community, so you will always find somebody to play PVP games (such as jackpot and coinflip) against.
You can play:
RustChance offers 0.50 flames ($0.50) sign-up bonus for new users. All you have to do is log in by steam, click "Redeem Code" at the top-left corner, and type "CATALOG" referral code.
There is also a faucet on RustChance, you can claim 0.03 flames ($0.03) every hour if you meet few requirements:
RustChance also offers bonus flames (it's currency) and cases for leveling up (by betting).