I'm starting this thread as a place to post info on legit methods of watching Doctor Who. Please keep in mind that this is not the place to post youtube links, dailymotion, or torrent links. Regardless of your personal feelings on the matter, this is a WhoNA forum and it's best (for all of us) that they stay in the BBC's good graces.
Probably the best place to get your Who fix. Their DVD catalogue has most of the BBC releases and they are usually available without any wait. Update: My streaming queue has marked all my BBC material as expiring on April 1st! I don't know what will happen after that point, but I'll post again as soon as I find out.
Netflix also offers a free streaming service for members which can be viewed on your computer or TV (using a xbox360, a $99 set top box, or some blu-ray players)
Netflix plans start as lox as $4.99 a month (although you'll want the $8.99 plan for streaming).
BBC America has released a limited selection of new and classic Who on Apple's iTunes store. These can be played on your computer, iPod, or TV via an AppleTV, iPod video-out cable, or by running a cable from your computer to your TV. Be advised - iTunes charges by the episode, not the story ($1.99 each).
Amazon has a selection of Who which can be rented for $1.99 an episode. It's a crappy deal and you don't even get to keep the file. I'm not even sure why they bother. The upside is that newer Tivos can tie into the service.
Lets you watch any episode that has been played on the BBC lately - only available in the UK though.
Ok, so you have to get off the couch for this one. Many libraries have sizable DVD collections (at least the bigger ones). Thanks to Doctor Who's traditional ties to PBS, this can sometimes be a great resource for getting ahold of older episodes.
That's all I'm aware of off the top of my head. There are new online streaming companies popping up all over, though, and the BBC seems willing to play along.... please let us know if you find anything that should be added to the list.