I always liked Peeron[1]. They have a great selection of the old instructions booklets scanned in, and half the time I've lost my old instructions or my nostalgia-driven Brickset order didn't come with them.
The quality for Peeron isn't always the best - it's very much still a Web 1.0 site, and so they aren't full-resolution, HD scans, just whatever was uploaded at the time.
But what I like about it is that it has pretty good coverage of what is for me "classic" Lego. The official site in the parent looks like it only goes back into the 90s, and is missing a lot of sets.
EDIT: read too fast the first time. It does kind of look like the same set of scans for the linked site, although they have coverage for newer stuff as well.
[1] http://www.peeron.com/