iTunes Music Store Player allows you to play multiple tracks off of the iTunes Music Store without having to manually double click and start playing the next one.

Usage example:
go to the music store (in iTunes 4) and do a search on "jazz". it will spit out a list of 250 songs. iTunesMSP will automatically play them all. do some work, and if you like a song, switch to itunes and buy it... ;-)
open a list of radio stations, like "Electronica". it will play each station for the amount of seconds you specify.
of course you can also use it on your regular play lists...

Functionality:
you can adjust the amount of seconds you would like to hear of a track.
you can check "Backwards" and it will play the list backwards.
if you check "Disable" iTunesMSP will be disabled.