Competition -- particularly smaller competition -- is forced to offer unique and advantageous features in order to best combat Steam's growing dominance over the PC gaming market. Not only is it unhealthy to allow one company total control over a market, it limits innovation and variety. That's what this list resolves. Remember, Steam's still out there for a profit -- their famous sales aren't just because they're nice people. Besides, Steam doesn't natively allow installation of games to multiple drives without external effort. For shame!
What to look out for
Looking for a new distributor is like looking for a new local gaming shop: You want someone that's reliable, friendly, has good policies, and isn't going to screw you over (or lock you out in the rain). The last bit correlates most directly with DRM, or Digital Rights Management, infamous for its limited number of activations and hidden registry keys. These implementations, along with the new requirement of constant internet connectivity, are an atrocious assault on the consumer in a failed attempt to "stop piracy" (which is an entirely different subject), so we need to cut the middleware out whenever possible.
Aside from limited-to-no DRM, you also want to make sure you own your games. Believe it or not, most of the time it is only a license that is owned to play - not the actual game and its content; this is true with Steam as well. You can be locked out at the publisher or retailer's whims, in many instances.
Finally, sales are an important aspect of any buying experience, so it is necessary to find sites that ask a reasonable amount for the products they sell.
Good Old Games
GoG has been around for a while now, and despite a few odd pranks and hiccups, they've finally found their place in the gaming world and have gained serious attention in the industry. Ignoring their namesake for a moment, Good Old Games also sells some new games (The Witcher 2, as an example) and offers them under the same policy as all of their "old" games: No DRM, no requirement for client-side applications, and you own the files. Do what you want with them -- format your drive? Fine. Reinstall your game, no activation limitations to worry about.
Good Old Games, of course, is best-known for their substantial selection of old (and no less valuable) video games; the company has worked tirelessly with publishers and developers to ensure older code will remain forward compatible, adapting games as old as Myst (1993) to natively work with XP, Vista, and Windows 7, removing the requirement for the ever-buggy compatibility mode.
GoG fits our criteria of being relatively benevolent, affordable -- averaging seven bucks a pop for older games, lower in some cases -- and DRM-free.
We've worked with GamersGate extensively over the past years as they've given us the most lenient policy for license sharing between our reviews editors. GamersGate, like GoG, attempts to minimize DRM as much as possible -- it hasn't eliminated it, but they've done a good job at decreasing the damage.
GamersGate offers similarly impressive sales as Steam, yet has the added bonus of no client-side requirements to play your games. You can always add non-Steam games to your list if you want to use the Steam Community overlay, but owning a game separate from an Internet-oriented service frees you from an obnoxious layer of bloatware. No one wants more Steams, Origins, and Xfires to run in the background, so GamersGate and GoG are both good options if you're not interested in more clunky interfaces.
Green Man Gaming
We've given Green Man Gaming some love for their PC rental model in the past, and it's time to give them another shout. Green Man Gaming is yet another digital distributor of PC games; they're available globally and offer one key feature that's not really found anywhere else: a trade-in option.
Unlike the previous two options, GMG does require a client-side application ("Capsule"), but it's understandable since they have the whole rental thing going on. The idea is that gamers can purchase a game on Green Man Gaming, play it, and use Capsule to indicate that they've become bored and would like to return the game. Green Man Gaming then reclaims their copy, you uninstall it, and you get credits toward another rental or purchase on their site.
The famous Humble Indie Bundle has spawned off an entire "humble bundle" series, where buying games often results in direct contributions to charities or other equally poor people, like independent developers.
Humble Bundles offer multiple games in a single package for ultra-low prices; recently, Mojang teamed up with Wolfire and Oxeye to group their three games, "Catacomb Snatch," "Fists of Resistance," and "The Broadside Express" (respectively) at charity-driven prices. According to the Humble Bundle website, nearly $500,000 was raised toward multiple charities, including Child's Play and the EFF.
You can sign up for the HumbleBundle newsletter to receive a blast next time they have a deal up. It's a great way to get exposure to emerging or indie games.
Impulse hasn't been my favorite option since Gamestop's takeover, but the service does still offer unlimited downloads, and for that I give them credit. If you're looking for services similar to Steam, including their community, friends, and chat interfaces, but don't want the install limitations, Impulse might be an option for you.
Like the others, they have frequent sales and offer a huge array of PC games, have no install limitations, and offer a client (if that's something you want). The downside is that, well, they offer a client.
With the absorption of Direct2Drive by Gamefly, Gamefly started targeting PC gamers more heavily and now offers downloads for purchase, as did D2D. Gamefly doesn't have the cool PC rental option that Green Man does, but they are a rapidly growing business that is founded on the console market (with several services that console gamers find useful), so they're not going away any time soon.
Of course, the best option is to buy straight from the developer -- not the publisher, not a middle-man, but the developer. Developers have a history of receiving the least amount of money from their games in the supply chain, often only getting paid after publishers can prove they've made a profit. Help out the makers of your favorite games and buy straight from their sites, no middle-ware and no crappy service acting as a go-between. Besides, most developers are really cool and would love you to send them an email to talk about their games. It's always nice to make friends with the creators of something you love.
Update: It's worth mentioning Desura as well, which is centered around indie games and mods.
-Steve "Lelldorianx" Burke.