I joined a new site the other day - it is strictly for ATC's. The folks there made me feel so welcome! They have tons of stuff to view and read and I am learning a lot and have gotten great inspiration for new work. They have a lot of challenges and swaps, big and small. The art is very diverse, ranging from collage to hand-painted works that are great. If you want to go check it out - site is now defunct but I trade here: www.ATCsForALL.com and IllustratedATCs.com
Let me know what you think!