Off the top of my head:
I'm no expert on this, but there are a few ways you can monetize a free online game (but it won't necessarily monetize well). What you have access to depends partially on the technology you are using.
If you are using Flash:
- You can always upload it to Kongregate (for example). They have ad revenue share and I believe you can add a tip jar (donate button) so people can give you Kreds which are worth real money. There are other sites like this.
- You can try to get a sponsor through FlashGameLicense
www.fgl.com (though I'm not sure if this is still an active market). The idea is that a sponsor pays for having their logo before/while your game loads.
Same thing applies if you are using Unity, I believe.
Note that is you are using Flash or Unity, you could set up your game to be on mobile (iOS/Android) with some effort and setup some other types of ads, and a donate button.
If you are in HTML5/Javascript/CSS:
- I know there are sites like FlashGameLicense that exist for HTML5, but I don't know them. Same concept as above.
- There are also sites like Kongregate, but for HTML5 games (again, I don't know them)
- You can simply set up traditional ads (Google, etc.) on the website you are hosting.
I think player donations can work, but it will always be a marginal percentage of your players who are willing to do this.
Finally, if you can transform your browser game into a native one easily, then you can sell it. Your work doesn't have to be free!
In all of the above cases, the amount of money will be directly proportional to the amount of people who play the game. This is not a trivial matter as it's one of the most difficult problems for indies, players knowing about your game.
Hope this gives you some ideas!
t