1. [link] The Signal and Image Processing Institute, University of South California (USC-SIPI): The USC-SIPI image database is a collection of digitized images. It is maintained primarily to support research in image processing, image analysis, and machine vision. The first edition of the USC-SIPI image database was distributed in 1977 and many new images have been added since then.
2. [link] This website provides free TOEFL iBT practice tests.
3. [link] The OEIS (On-Line Encyclopedia of Integer Sequences) is a database that collects number sequences.