After
The Weber Show died a horrible death
NBC began airing a rerun of their highest rated sitcom for the week in the 8:30 slot. For example, one week a
Friends rerun would air at 8:30, whereas next week a
Frasier rerun would fit into the timeslot.
This practice continued until the 2001-02 season (barring nights when the entire lineup was pre-empted) when a little show called Inside Schwartz premiered in the 8:30 slot and, like everything else NBC has left in the timeslot to die, was largely ignored by the audience. The show was dead before 2002 and the 8:30 rerun slot returned.
For the 2002-03 season NBC seemed to wise up and realize that just because a show followed Friends doesn't guarentee it'll be a hit. The network scheduled their break-out show Scrubs at 8:30 where it continues to do well. The network cancelled Just Shoot Me suddenly in 2003, leaving the 9:30pm timeslot open for a new comedy in the fall. The rest of the evening's schedule remains unchanged.
For the 2002-03 season NBC realized that this would probably be the last year that they could squeeze obscene advertising prices for the 8:00 slot since Friends was 99% sure to wrap up for good at the end of the season. Therefore Scrubs doesn't actually air until 8:32pm. NBC claimed that Friends needed the extra time to wrap up loose ends in preparation for the series finale and therefore would run "creatively long" all season. However, the actual running time of each episode remains unchanged, but there are - surprise! - two extra minutes of commercials in the broadcast. Likewise, the actual running time of Scrubs is two minutes shorter and it runs the same amount of commercials that a standard thirty minute broadcast would garner. Of course the series was renewed for one last season and will probably run long again in 2003-04.
In 2003 NBC brought the American version of the popular Britcom Coupling to the 9:30pm slot... where it died a quick death, being sent to "hiatus" after only three weeks. Friends, Scrubs, Will and Grace, and ER remain in their respective timeslots.
One last little footnote... several of the Must See TV programs all exist in the same universe. Characters from Seinfeld, Friends, and Mad About You have appeared in other programs in the group (such as Helen Hunt's appearance as Jamie Buchman on Friends). Likewise, Cheers, Wings, and Frasier are all a part of the same universe (Kelsey Grammer as Frasier Crane has appeared on all three shows). For a time in the 1990s NBC would also air "theme nights" in which events in the 8pm show would carry over to the other programs following, such as when Paul Buchman caused a blackout on Mad About You and then the rest of the shows that evening took place during the darkness (On Friends Chandler Bing was stuck in an ATM vestibule and on Madman of the People the characters were trapped in the subway. Seinfeld did not participate in this ongoing plot event).