According to this GI story, Yahoo! and Microsoft both rejected Steam after Valve pitched the pre-launch concept.
"You know, we went around to Yahoo, Microsoft... and anybody who seemed like a likely candidate to build something like Steam," said Valve marketing boss Doug Lombardi.
"We basically had our feature list that we wanted. We wanted auto-updating, we wanted better anti-piracy, better anti-cheat, and selling the games over the wire was something we came up with later.
"We went around to everybody and asked 'Are you guys doing anything like this?' And everyone was like 'That's a million miles in the future...We can't help you.'"
Steam's 14 million accounts say otherwise.