Is there any services similar to steam which allows you to send a message to someone who is playing another game? I would use steam but you need to buy a game to add friends and my friend does not want to buy anything on steam.

There's XFire. I haven't used it in over four years, so I don't know how it changed. It made Oblivion crash for me, but your experience may vary.

I haven't used Steam for a while, but I think I vaguely remember being able to add people without having bought any games.

Steam requires you to have purchased a game to add friends now. I would but my friend won't so Steam is out of the question. I will try out this Xfire, I doubt it will crash my computer is a tank. If you know any more please post I will not be home for a few hours anyway. Thanks.

Xfire works with both Steam and nonSteam games. Some have issues with it though, like the original FEAR game but for the most part they work fine.

As others have stated there is Xfire and there is also Raptr. I haven't personally used Raptr, but I've heard quite a bit about it, and it sounds like a really solid program.

Couldn't he just "buy" a free game? Like Alien Swarm.

Nope. Must be a monetary transaction.

Only one person needs to have bought a game. You can still accept invites if you haven't bought any games. I chat with my friend over Steam all the time even though he hasn't bought anything for Steam.