Hmmm....I've got lots of collector's stuff (including Empire of the Petal Throne, Metamorphosis Alpha, The Morrow Project), but in terms of RPG items that I got when they came out...
D&D Basic Boxed Set from 1978--the one with the blue book, dice (not those chinzy chits) and module B1: In Search of the Unknown
Classic Traveller Boxed Set (1977)
um...what else...
Module D2 & D3 (the 1978 monochrome versions)
Module T1 The Village of Hommlet (1979)
The Official Advanced Dungeons & Dragons Coloring Album by E. Gary Gygax (1980)