www.nintendo.com -Nintendo of America always offers semi-up-to-date coverage, with exclusive pictures and previews. 
www.nintendo.co.jp  -Nintendo of Japan gets their news before anyone else.  They already have pictures of the production model on their site.  Great if you can read japanese :)
www.nintendojo.com/gbcdojo -GameBoy (oh boy!)-  Gets the best non-official news, rumors and pics.  The new location is at Nintendojo, and I will update the link accordingly as soon as I write down the address.
home.hiwaay.net/~jfrohwei/gameboy -Jeff Frohwei's Gameboy Tech site-  Great if you want to know what makes the GB tick... 
http://gameboy.s-one.net.sg EAGB- Good news and tech site for European and Asian gameboy owners. Exclusive pictures.  The worlds best GB and GBC page, with more exclusive content than you can shake a stick at!  Also, these guys sent me cool stuff, so give them a visit!
http://come.to/Gameboy Toads GB Headquarters:  Has some game reviews and history, a good site overall.
http://www.dragontooth.mcmail.com/gbmono/  The Gameboy Monolith- An amazing site with all sorts of great stuff- ratings for over 300 games, news, message boards, chat, walkthroughs etc.  Very well put together!
http://come.to/DmgIce  The DMG ICE site has a whole bunch of good info, some exclusive stuff too, and just too much to list here.  Visit this site if you have a lot of time to spare!

If you have a question I might be able to answer, mail me at modfactor@hotmail.com