Steam

I have used it since a few days after release (Sept 13, 2003) because I was playing Counterstrike. It made updates and finding play servers so easy even though it did have a rough start with connectivity. Honestly, it was better than whatever we had to use prior even with the issues.

Once they sorted out the server issues and started adding non-valve games it became even more useful and we end up where we are now.

They are currently still on top because of being a solid storefront and the other things I listed.