Right now, Steam is free. Which is lucky, since for all its benefits, it can still be a pain in the ass! Luckier still, Valve have committed to keeping it that way (the free part, not the pain/ass part), stating that not only are they planning on keeping Steam free, they've got a perfectly valid reason for doing so:
We will not charge for The Steam Community and its features. Never even entered our mind. Part of it is to address the fact that when folks first bought Half-Life 2, Steam didn't offer many reasons to stick around. It didn't do enough for customers, and it didn't justify itself. So I suppose we're paying off a debt now.
Awfully big of them to admit that. Well played, Valve. Valve 'Will Not Charge' for Community Features [1UP]