1) Faucet Earnings
This is the most common method used by bitcoiners to fill their wallets. But this is low profitable method considering the data cost and time. There are many type of faucets including fun games etc. Here are some of them.
2) PTC sites
These are the websites that pays you using bitcoins for clicking ads. Cost huge data and low profitable method. Some ptc sites are below.
Listia is a online shopping website where buying and selling is done by listia credits. You can earn credits in many ways. Important thing is at the signup and just after you complete your profile you have earned at least 5000 credits. So now you can bid on items (bitcoin) and earn around 10000 satoshi (.0001 btc) just for 100 credits. Now you can see how much profit you can get from this.
SignUp from this link to get your bonus credits
4) Bitcoin Investing
This is a highly risky method that I never recommend for any bitcoiners to follow. But this is a effective way to increase your existing bitcoins within few days or loose every thing you have. Most of the bitcoin investment websites are scam. They take your bitcoins promising you high profits and get disappear with your coins. There may be legit investment sites that you can trust. But it is very rare. I'll give you few legit sites.