I've been using Steam for almost a year now, and I own 70 games on my Steam account(no joke). I have a close group of Steam friends and have created a gamer clan composed of 370 members in my public group and 12 members in my private group. I've obviously been content with Steam being my main gaming platform, and I didn't feel the need to look into other digital distribution services...
...until I found out about Good Old Games(http://www.gog.com/; GOG here on out) by chance while casually browsing the internet. GOG(if you don't already know this) is a distribution service that provides old PC games at ridiculously good prices, usually $5.99 to $9.99 for individual games, sometimes higher for game packages. I was surprised at the great catalog of games, including the older Fallout titles and games like Sacrifice that I wanted to play, but never had the chance to. My father owned Sacrifice for a while, but he could never get it to run on his PC(OS issues). I was very interested in the game at that time in my life, but I could never play it, so you could imagine my excitement when I learned that GOG carries it for only $5.99.
At close examination of a GOG official post about the service, I saw things like "No DRM!" and "you can download the games you buy to as many PCs as you want; it's your game, you bought it!" I didn't completely comprehend this for quite a while. With Steam you need to keep the Steam program running to play your games. It has been called by respected gaming journalists the most intrusive form of DRM possible, as well as the best thing to hit the PC gaming market(by the same people, no less). I'm completely alright with Steam running in the background while I play games; In fact, I'd rather use this safe form of DRM than not. I certainly don't want pirates to damage the PC gaming market any more than they already have! It just appeared to be the most logical thing to do for a digital video game publisher. Now with GOG: No DRM? As many PC downloads as you want? Isn't that like selling 20 copies of a game for the price of one? I was sure they had a platform for their games to prevent mass pirating, but my assumption was wrong. They provide ridiculous freedom and great service. What's not to like here?
Direct2Drive(http://www.direct2drive.com/)is one of those services I referenced in the first paragraph that fails miserably. You buy a game, and then are limited to a certain number of downloads. DRM comes with quite a few of their games, the service is terrible, and the prices are just as bad as retail, if not worse. I may never use this service; not too interested anyways.
A few weeks ago I read an article on Demigod and remembered that it was the cover story on one of my old Games for Windows Magazine issues(my favorite PC gaming magazine; now sadly dead along with EGM), and I was automatically excited for it. When I pulled up the Wikipedia page on the game, it said that Demigod works with Impulse, Stardock's digital distribution service. I had no idea that there were any services that could potentially compete with Steam! It was a bit of a shocker. I decided that it was very interesting and quickly downloaded Impulse.
Now, here I am. Impulse doesn't have all of the features as Steam, the interface is inferior in some regards, superior in others, and the games provided aren't very exciting. The only games I'm really interested in with Impulse are Demigod, Sins of a Solar Empire, and... well, that's about it. =/
What's so good about Impulse, then? Well, the freedom Impulse provides is nice. GOO looks like a really big innovation Steam would benefit greatly from, selling software to Impulse users in the same browser as the games sold on Impulse is nice, and the fact you don't need Impulse running at the same time as your games is very interesting to me. It looks like Impulse is being built upon, and that it will get better as time passes, as Steam has over the years. I wish Impulse and Stardock a great future. Luckily, I'll be along for the ride, and I'll be able to examine the development of a digital distribution service. 
This article was partly written(typed?) for Steam users in Clan Syphon, and I'm just expressing my views on digital distribution as a whole here. Hopefully this will cause a few PC gamers to look into alternate distribution services. That's the main reason I spent an hour typing this. =O Sorry if I said anything inaccurate. I hope I didn't.