Some photos of a visit to the Valve "office" in 1998 made it online:
http://imgur.com/a/YxcSu
In particular this one was interesting and relevant to tf2maps:
This was probably WorldForge back then, the precursor to Hammer, but it's funny how we're using basically the same program today.
http://imgur.com/a/YxcSu
In particular this one was interesting and relevant to tf2maps:

This was probably WorldForge back then, the precursor to Hammer, but it's funny how we're using basically the same program today.