Yu-Jyo - This site is a great resource for information about all the differences between the Japanese series and the 4KIDS dub of it. Actually, I'm pretty sure it's the ONLY source. Check it out. NOW. :P
http://www.janime.info - This place is another great resource for Yu-Gi-Oh. There's so much there, I'm not even going to try to describe it all. Just go there.
Michael's Yu-Gi-Oh! - I'm not sure exactly what's there, but I'm sure it's a great site. And LOADED with screen caps of the US version.
Xena's Yu-Gi-Oh! Screen Caps - Just the opposite of the above site. Xena's site has loads of screen caps from the Japanese version of the series, as well as other series too.
YuGiOhRealms - Best YuGiOh online community I know of.
DeckRevolution - My cousin keeps telling me I need to check this place out, but I haven't had the time. Maybe you should though.
XeroCreative - Another great online community, only for some reason it won't let me register. Maybe it's just my computer fucking up. Go here to download the infamous YVD(YugiohVideoDeck) and www.scrollrack.com to download the additional files for the program, which is basically all sets made after and including Flaming Eternity.
Other
Masters of the Obscure - Hardcore Marvel fan? Then this is THE place for you to be. Loads of great resources on all those characters you've probably never heard of.
UncannyXmen.net - A great site for loads of great info on Mutant this and that. Best database of info on the net regarding everyone's favorite mutants, as well as many Q&As with the creative teams that write them.
Mutant Liberation Front Forums - Looking for a place to talk about ANY comics you want with people who like them just as much as you? Well this is just about the best online community for that you'll find.
Mausland - Maybe your bag ISN'T yugioh or comics, and you're obsessed with games. Personally, I HATE video/computer games, but I'm betting you don't. Go here and play a multitude of cool games designed by Wiesi, or just chat with lots of cool people on Mausland's message board.