They sell a new shirt M-F until it sells out. They will also allow you to buy shirts that you might've wanted before but was sold out.

I was thinking about picking up these
http://shirt.woot.com/Friends.aspx?k=3630
http://shirt.woot.com/Friends.aspx?k=3625
http://shirt.woot.com/Friends.aspx?k=3622

They're $15/pop when they sell out and get put back up, but regularly they're $10.

Link for daily tees is Http://www.shirt.woot.com and for old sold out tees
http://shirt.woot.com/Blog/Default.aspx