Burke Library provides access to A to Z Maps Online -- thousands of maps royalty-free to view, download or print (in several ready-to-use formats, including Powerpoint- and PDF-ready sizes).
The Perry-Castaneda Map Collection at UT/Austin includes an up-to-date topical selection of maps related to current events. This site also includes an extensive clearinghouse of online maps at other institutions.
The Library of Congress has custody of the largest and most comprehensive cartographic collection in the world with collections numbering over 5.5 million maps, and much more.
The David Rumsey Historical Map Collection has hundreds of fascinating examples of old maps.
The American Geographical Society Library Digital Map Collection currently contains over 1,000 maps, ranging from early maps of Asia to historical maps of Wisconsin and Milwaukee, and other American cities, states, and national parks.
Produced by JISC, we have Old Maps Online, which will "act as a central repository to a vast collection of maps held by institutions across the globe." When you initially access the site, it initializes to your location.
The Harvard College Library Map Collection boasts "one of the oldest and largest collections of cartographic materials in the United States with over 500,000 items."
The British Library sponsored an exhibition of Magnificent Maps in 2010 which are still accessible online.
A Vision of Britain Through Time provides a nice set of historical map images alongside other data of interest to scholars.
Stanford University hosts the Spatial History Project which is "a place for a collaborative community of scholars to engage in creative visual analysis to further research in the field of history." Their Gallery includes a number of stunning animated maps and other tools.
The ECAI Cultural Atlas Portal showcases cultural atlases searchable by countries, regions, or cities.
The Globalis Interactive World Map provides gazetteer-like information in an on-line, interactive format.
The World Mapper is a collection of world maps, where territories are re-sized on each map according to the subject of interest.
The Poverty Mapping site "provides spatial representations of poverty assessments" worldwide.
Someone decided to visit and take a picture of each of the latitude and longitude integer degree intersections in the world; you can join up and contribute at the Degree Confluence Project.