The Brickfactory - LEGO Instruction Sets Archive
This site has hundreds of scans of instructions for LEGO sets. You can select the instructions for sets by theme, year, number and name.
While it may not be fully comprehensive, the sheer amount of material makes it a treasure trove for LEGO lovers. It’s also a great resource for people with missing instructions for their sets. You can also send scans of your own instruction sets, to the site.