Intute : sciences websites browser
Intute: Science, Engineering and Technology
In fact, it is a yearbook of websites. Their database contains 34449 records.
Intute: Science, Engineering and Technology offers a free, easy to use and powerful tool for discovering the best Internet resources for teaching, learning and research, covering the physical sciences, engineering, computing, geography, mathematics and environmental science
The required level may be a little high, but I think it might be quite interesting...
The database is said to be updated by universities and scientific institutions.