By far the most important and useful web site I use is Flash Game License. As it says below its logo it is "The place to buy and sell Flash games.", so it's a marketplace. It's needed because of the structure of the Flash game industry, with many buyers and even more sellers, of games worth so little that they are not worth advertising or heavy promotion.
Flash Game License sells games via an auction process, although different from most auctions in some important ways. Bids usually include a price but may include other terms such as additional sources of revenue such as performance-based or requirements for changes, usually at least adding the buyer's (or sponsor's) branding.
Auctions do not have a deadline. They run as long as the developer wants, until they get a bid they are happy with or withdraw from the auction, though there are a couple of ways developers can force a quick end to the process. There is no up-front cost to developers: Flash Game License take a cut of all completed sales, paid after the auction is concluded. This also means it's in their interest to see the game sell for as high a price as possible.
If that were all the site offered it would be reason enough to use it. But as well as a robust and flexible auction system there is:
- A free pre-review service for games by experienced FGL staff
- 'First Impressions', anonymous mini-reviews by real users (the first few are free, after which they are very cheap)
- Feedback from other developers, often very high quality, free or for similar feedback in return
- An integrated distribution service, FlashGameDistribution
- Forums and chat: many sites offer forums but these are distinguished by the quality of the contributions, as so many participants are experienced developers
- In particular forums for GameSafe (back end services such as microtransactions and level sharing), CPMStar (in-game advertising) and Kindi (makers of secureSWF). Again these forums are especially useful because of the other developers that visit them
If Flash Game License did not exist someone would have to create it. No-one else has done so since because I doubt it could be done better.