Just in case people in the US were feeling left out due to the article last night about Ordnance Survey in the UK providing GPS differential data, people with readings in the USA can go to CORS, the Continuously Operating Reference Station site from the National Geodetic Survey. As with the Ordnance Survey systems, this is really only useful to dual-frequency users, although it is planned to have OPUS (see below) available for L1-only users in the future.
CORS provides data in RINEX format for GPS point referencing and covers over 650 stations in the US and its territories. There is also a link to Cooperative CORS, a companion program where qualified volunteers provide additional data to the system. Both systems use fixed locations and dual-frequency receivers (L1 and L2) to determine coordinates.
To make the data even easier to use, the National Geodetic Survey also provides access to OPUS (the Online Positioning User Service) which processes your RINEX data against the data from CORS and Cooperative CORS and provides coordinate information in a variety of formats. The minimum data gathering period is 2 hours and the maximum is 24 hours. Data must be in RINEX format.
Anyone interested in becoming a part of Cooperative CORS can check out the Becoming A CORS document that is posted on the NOAA site.