- Taking shares in a mining pool
Not likely to get positive return.
- Lending to a fellow bitcointalker
Highly risky if there's no proper collateral.
- Taking shares in the working capital of a betting site (funding the bank)
Highly risky. You have to trust the operator will never steal or just run with all the funds.
Conclusion:
The only valid way now is starting your own business with BTC, or just put it in cold storage.