The most important database for LEGO. The inventories of all Lego sets ever produced, and the possibility to search and order every available Lego piece in thousands of shops: Bricklink

The second most important database for Lego. A lot of fotos and reviews of nearly all LEGO sets ever produced, and the possibility to search and order every available Lego piece in thousands of shops: Brickset

A lot of ideas for mechanics, the books of Yoshihito Isogawa.

Applications: Instructions for LEGO (Apple App Store)