Only mods I know you can get through Steam are mods for their HL2 games - and even then, they are oldie-mods (aka mods that were in their prime two years ago). After they introduced those mods on there, they kind of abandoned the modding idea, for the moment.
I wouldn't be suprised though if sometime in the future they did a big modding push, as perhaps a way to draw in more customers or compete against Desura. Currently, there isn't a need for that on Steam's part. But eventually I feel there will be.