Awesome site for fans of anything GM! Click on the "Forums" button at the top to find answers to all your GM questions, and some really great people.
Just as its name implies, this site is dedicated to full size Chevy and GMC trucks and SUV's. They deal mostly with newer models but there is a section for vintage trucks as well.
Now days this site's name is not what it is. They now include anything from 1947 to current as long as it is GM made!
Now for something not GM made... Dodge Forum.com! This seems to be a great forum for information on Chrysler's truck manufacture. I have gotten some great info and help off of this site for my 2008 5.7L Hemi Ram 1500
Found on Jalopy Journal.com is the H.A.M.B., stands for Hokey Ass Message Board, which is filled with some of most knowledgeable hot rodders on the planet... yes, planet as this site is more than just a U.S. thing!!
Not related to hotrodding but a freakin' awesome forum for motorcycles both custom and not. Check it out for sure!
Though not about hotrodding this forum deals with something that anyone who works on vehicles needs... a garage/shop!
Another forum dedicated to the life-style of hotrodding
And another one that I have found particularly useful is Old GMC Trucks.com. Tons of information these awesome trucks.