The History Highway: A 21st Century Guide to Internet Resources (4th ed.) is a handy directory of history related websites.
There are more than three thousand annotated entries organised by geographical area, historical period, and subjects such as history of science, urban history and so on.
It is available in the Arts Reference Collection in the General Library (call number 025.069 T83 2006). An accompanying CD-ROM has a PDF copy of the book with live links to all the websites.