There's a few that I remember at least being spread around at school right when I was in elementary school and Pokemon was new to the States. The most obvious being the truck rumor with Mew underneath or catching Mewtwo with a Pokeball with DOWN+B (boy the number of times I fell for that one)
A few years later I remeber news about PSONE (the slim redesign) and because I didn't keep up with Sony then, I heard from someone that PSONE didn't require a modchip to play burned games.
When rumors of PS2 came out, I remeber stuff like the ability to load a DVD and watch it while waiting for the game to load or expecting Toy Story quality animation from this powerful hunk of a system (though now, the watching DVD thing while loading a game sounded ridiculous now). I remember hearing that it could even make the graphics of PS1 games better (as a kid, you didn't really understand that stuff wasn't true you just went along with it). I remember seeing footage of PS2 gameplay and thinking "Wow, how can graphics get any better than this?" but this was way before news of XBOX or GC came out.
I saw a contest once for Fruit by the Foot once and one of the prizes was Game Boy Color but on the screen, they'd edited it so that you saw the peace sign Mario does in Mario 64. This led me to believe that a Game Boy capable of N64 games but in the form of GBC was happenining. And that was before news of GBA ever happened.
Lastly, I remember rumors of when PS3 was being prototyped and it was about 6-8 PS2's hooked up for crazy power. Depending on when you heard it, kids loved to exaggerate it.