I miss several shows that people have already listed, so I'll go in a different direction.
I miss getting up on Saturday mornings and watching cartoons in general. I really loved the Nintendo themed shows like Captain N the Game Master, even though I knew a lot of elements in the show were horribly inaccurate. I also remember catching a different version of the Super Mario Bros. Super Show on Saturday mornings except that it was Mario World themed. Although this isn't exactly a rare thing, I did catch old Warner Bros. cartoons (Bugs Bunny etc.) with my mom on ABC 7. God...this probably isn't helping me get away from missing my childhood horribly.
Here, for those of you who grew up the way I did...
http://afterthesemessages.ytmnd.com/
*sigh*