If Half-Life was never made, we would see far less adventure/shooters (but seriously, name one other good one). If Counter Strike was never made, the PC shooter would be based around the Call of Duty and Battlefield series instead (which it pretty much is now (and I think Counter Strike was a third-party mod of Half-Life, so we may have seen it the form of a Quake mod anyway). Portal would have been made regardless, though it's argueable about if it would have been as good (but I have yet to see a PC game that screams "I want to be like Portal!")If Steam was never created, people would just buy their games off of one of the many other sources or just buy it at Best Buy like a normal person, and they would use Xfire, forums, and in-game game selection to play with friends.
I don't think Valve made as big of an impact as you think.