DS2 is currently loving our blu rays of "The Universe". I think lots of similar types of science shows are inherently interesting to little ones. After the first episode he drew a cute little solar system and excitedly brought it to me.
As for PBS shows aimed at the younger set, these have impressed me as being decent: "Martha Speaks" and "Wordgirl". "Super Why!" is okay too, mostly for drilling on early reading skills. I believe someone else here has recommended the show "Between the Lions", but I haven't seen it yet. ETA: Oh, yeah, and "Word World" is often recommended.
