CS 1.6 was exclusive to Steam; I remember waiting quite a while to register because I didn't like the idea of Steam but ultimately caved (still have a 6 digit steam id, which used to sort of but not really mean something). The server browser in CS 1.5 and below was built into the menu's on the application.

This is indeed the Steam UI, not Counter-Strike. This was also back when they had games like Go built in that you could play with people on your friends list.