Not sure if this is legal or not, but PeekVid is offering quite a few (slightly older) episodes of popular television shows online. There aren't any annoying ads and you can watch them right in your browser (maximize the playing window to go full screen). The site is well organized; it appears that the content shown is simply garnered from usenet postings using Guba.
There are, of course, legal alternatives; Channel Chooser (70 channels) and wwiTV (1000+ domestic and international channels) are the first that come to mind. Makes me want to run ethernet to the television upstairs (wireless is just too slow)... but I'm going to wait until we know for sure where we are going to be living in 3 months.