As a raster analyst, there are a lot times I try to “fix” many types of raster data from various online sources. If you’re going to get data from elsewhere, there are many reliable sources available for GIS data, some websites that charge a fee, and some that are really well-done free clearing houses and data centers.
A few of my favorite national and state/regional sources for GIS raster data are:
Geo.Data.gov - http://geo.data.gov/geoportal/catalog/main/home.page
USDA Geospatial Data Gateway- http://datagateway.nrcs.usda.gov/
USGS Seamless – http://seamless.usgs.gov/
GeoCommunity – http://data.geocomm.com/
Great Lakes Coastal Planning – http://aqua.wisc.edu/cpr/Default.aspx?tabid=85
North Carolina One Map – http://www.nconemap.com/
Minnesota – http://deli.dnr.state.mn.us/data_search.html
California – http://atlas.ca.gov/download.html
ArcGIS Online – http://www.arcgis.com/home/
National Hydrography Dataset – http://www.horizon-systems.com/nhdplus/data.php
Thanks for all of the contributions in the comments!
Check out the Resources for Raster Data page on Wiki.GIS.com for additional links and an updated list.
Jeff S. - Raster Support Analyst