Here are some links to pages I've found extremely useful...
AR15.com Good source for everything AR (or many other guns) plus a gathering spot for information sharing.
http://www.dpmsinc.com/ Place to buy nice guns, but if you go to their customer support section, they have some nice schematics.
http://www.brownells.com/ Good place for parts and how-to's on building your AR
http://www.midwayusa.com/ My favorite place to shop!
www.rockriverarms.com/ Another good place for parts or entire assemblies
mapartsinc.com/ Another favorite shopping site
I'll keep adding more as I remember/come across them
