Peeron Catalog and Instruction Scans
While Lego experts are probably well-acquainted with Peeron.com's extensive database of Lego models, it's a fantastic place to start for the Lego layman. In addition to housing building manuals for a good chunk of old sets and the catalogs released worldwide, they provide current Amazon.com Lego sales, Lego associated RSS feeds and a parts tracker that keeps hold of the pieces you have in your collection.
Peeron stirs up fond memories of your first sets—mine being the Space Command Center (493) and its supporting cast, save the Space Shuttle (442), which mysteriously flew off the shelves of the Clark AFB PX. I vaguely remember being very pissed off about that.