Most topographic maps of the world are available for sale only. Exceptions are worldwide topographic map series by US mapping agencies (some are restricted.) To find world topo maps, try the followings:
Perry-Castañeda Library Map Collection - Topographic Maps
The University of Texas Libraries scanned print maps and made them available on the web.
The USGS topographic maps are available for downloading from the following selected websites:
USGS Map Locator & Downloader
USGS's official topographic maps (7.5'=1:24K, 30'x60'=1:100K, 1=1:250K) download site. Maps are in the GeoPDF format.
Libre Map Project
The Libre Map Project houses map image files (TIFF), GIS Data and documentation. Its data includes 24k scale USGS DRG data, SVG coundary files, TIGER/Line data and GeoNames. Users can search by state, place name, and feature, and data is available in tiff, tfw, fgd, and TIGER/Line formats.
Historical Topographic Maps
The followings are selected sites which offer historic topo maps.
MapTech: Historical Topographic Maps (US Northeast)
A collection of historic USGS topographic maps of Northeastern States (so far). See also the University of New Hampshire's original historic USGS Maps site - UNH Historic USGS Maps of New England & New York.
Topographic Maps of Illinois
Historic topo maps of Illinois of various scales.
Broer Map Library
Check sections with names like "[State Abbreviation] USGS Maps"