Instructions. LEGO Customer Services publish instructions at their Building Instructions portal at LEGO.com. This page provides a means to search them easily. Note that we don't host the instructions, we just link to them on LEGO's servers.
Another website that wasn't mentioned yet: LEGO MOCs (Custom LEGO Sets) with Building Instructions (rebrickable). Here you can find a ton of MOCS, the instructions for which you have to pay are clearly marked.
Worldbricks - LEGO instructions and catalogs library
lego instructions free download - Cool Instructions for Lego, ArtiFex Creation - Interactive Building Instructions & Videos for LEGO, Instructions for LEGO - Help To Create New Toys, and many more programs