When you first open steam, steam will load a popup window featuring any latest updates to a program or a new game. This is also included on the FEATURED games on the front window of steam on the store. (it cycles the latest games or those with new features).
I've seen games do the "FREE Weekend of Civ V" or "FREE Weekend of Left 4 Dead"...
This will show on both Website (non-popup) but on the rotating marquee, and on the Steam App both popup and rotating marquee. Good advertising.