I forget where I first saw this linked, but CBS is streaming a bunch of the early seasons of Star Trek: TOS and The Twilight Zone* up on their website for free. For those who've never watched The Twilight Zone before, a bunch of old Simpsons episodes are about to make a lot more sense.
William Shatner appeared in three of the five shows they have posted (though not necessarily in the seasons currently up now). I find his omnipresence oddly reassuring.
*Also MacGyver, Hawaii Five-0, and, uh, Melrose Place, if that's your thing.