The most important features that I view Steamworks brings to the table. I got this info off from Steamworks main page.
Persistent identity framework
Achievements, leaderboards, profiles, and avatars make your game come alive, building your community and providing a strong incentive to play more and recruit others.
Steamworks' multiplayer back-end is powered by robust matchmaking and lobby technology. The same technology that drives the quick and accurate match-ups in Left 4 Dead can be used in your game. Works with both peer-to-peer and server-based games. Steamworks' built-in voice functionality enables players to work quickly to strategize their next misson.
The Steam Community
Your game becomes the talk of the town as part of Steam’s large and connected Steam Community. Friends will see friends playing your game and will be able to organize matches, compare achievements, and talk about the next match, sequels, favorite parts, or their favorite villain.
Sell additional content from within your game to the customers who want it most. Steamworks provides true in-game DLC, allowing customers to select, buy, and use DLC — all without leaving the game. Additionally, using Steamworks' DLC does not close off your other channels. You are still free to sell the content at retail, either with other online sites or through the Steam store.
Access from any computerWith true cheat detection, keep cheaters from ruining it for the rest of us. Extend the life and sales of your game by making sure that the game plays as you intended it to when you shipped.
Voice chatYour customers can sign in and use their game from any PC or Mac. Access to their games is based on a customer account, not tied to a computer. Plus, with built-in offline mode, your customers can play on laptops when traveling — or anywhere else.
Steam Anti-PiracyWith built-in voice chat, players can talk to each other both inside and outside the game.
Beta-testingSteamworks' anti-piracy suite combines three approaches to anti-piracy: (1) Custom Executable Generation, (2) Retail Encryption, and (3) Valuable Platform-Dependent Features. These work together to provide a seamless end-user experience, protecting your game and giving real value to customers.
1. Custom Executable Generation
Custom Executable Generation creates a unique build of your game for each user, making it difficult for any one user to share the game with any other user. Each individual copy of a CEG-protected game is only playable by the Steam account authorized to access it. CEG is transparent, and does not impose limits on users. It lets users access their content from any hardware, and allows unlimited hardware configuration changes without the content becoming unplayable. In fact, no changes are made to a user's computer for CEG to work. Instead, CEG works in tandem with Steam authentication, enabling content access based on user accounts, not arbitrary hardware-based "rights-management" restrictions.
2. Retail Encryption
Protect your day-one release by shipping encrypted media to stores world-wide. No need to worry that your game will leak from a replicator or stock room - your game stays encrypted until the moment you decide to release it.
Encryption technology also allows you to preload your game to users on Steam. Download encrypted data weeks in advance so that players have access to your game the moment it's released, increasing player numbers and satisfaction during the critical first days.
3. Valuable Platform-Dependent Features
Customers won't want to pirate a game that's connected to 20 million gamers and a feature-rich platform. Features like Steam Achievements, Anti-Cheat, Auto-Updating, and Steam Cloud simply dont exist outside of Steam.
Furthermore, constantly updating your game with upgrades and content leaves the pirates in the dust they are relegated to a featureless game with no community of players.
Free Weekends and Guest PassesRun secure public betas of your game. Hold public betas and choose from one participant to thousands, if you like. The timing and scope of the beta is up to you.
Real-time sales dataHold events like Free Weekends (or any length play period) to entice customers and promote your game. Then securely and efficiently turn the promotion off at the end of the period. Also available is guest pass technology that allows you to promote your game through your community. Let friends give free, limited-time passes to their friends. This is proven tech that has been used with our games and third parties.
Log on to your personal sales and data tracking site and see your product’s statistics up-to-the-minute. Don’t wait for weeks to find out where and how your game is selling. Real-time, worldwide activation and use reports give you the detail you need to make informed decisions about marketing and retail conditions when they need to be made.
As a person who owns 64 games on Steam, I really like Steam as a whole. I'd really like some of the Steamworks features such as the server browser, stats, and right clicking on a friend whose playing ArmA II to instantly join their server.
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