If you haven't noticed yet, CBS has added the entire Star Trek TOS collection to their "classics" area for online viewing. One catch: You'll have to watch a few commercials.
For those of you who don't want to watch via the computer, some of TOS is also available via Netflix Instant Watch. And if you don't want to watch alone you can join GWCers over on the forum for their Tuesday-night TOS frak parties. It looks like they're on episode 1x03 The Corbomite Maneuver right now.
All Trek TOS [CBS Classics]
For some reason, they do not include Assignment: Earth
Unavailable outside the US, alas…