There is the Windows Store, but with its limited OS support I'm not sure how successful it will be, but I can see the need for it. In fact, why doesn't Steam itself extend its service to include software other than games? The only downside I see is that software developers may be reluctant to publish there since it would add a requirement to their software that Steam be installed on the user's PC. For software developers, the less the requirements, the wider the audience base.
What do you think?