Prospekt is a fan made sequel to Half-Life: Opposing Force that Valve is allowing to be distributed through Steam. Priced at $10 (USD), the release is set for February 11 and will return players to City 17 from Half-Life 2.

You can actually pre-order it right now for a 10% discount.

I suppose if this keeps up, Valve will simply get to the point where the fans themselves make and release Half-Life 3 through Steam.