The Global Positioning System (GPS) has revolutionized geodetic positioning providing an inexpensive means for a wide variety of non-traditional users to determine their location. Although the DoD provided civil signals enabling precisions of tens of meters, users discovered early on that reference stations providing differential correctors in real-time or for post-processing would improve accuracies to 1-2m. When the carrier phase data from reference stations is combined with the user's data, accuracies on the order of 1cm are routinely achieved. The National Geodetic Survey (NGS) has developed an online service that allows users around the world to submit their data for processing using NGS software on NGS computers, delivering cm-level accuracies via email within a few minutes. Developed by the Geosciences Research Division, the Online Positioning User Service (OPUS) has processed over 600,000 solutions in its 6 years of operation. Using as reference stations the Continuously Operating Reference Station (CORS) network, which now numbers approximately 1200 precisely determined GPS tracking stations, OPUS has provided fast and accurate positioning for surveyors, engineers, GIS users, and a variety of remote sensing applications. Almost anyone using GPS may be able to efficiently enhance their positioning accuracy within the National Spatial Reference System (NSRS). Due to its popularity, OPUS is evolving to include publishing user submitted positions in the NGS Data Base; automated processing of network data; positions and trajectories from inexpensive navigation grade receivers; and rapid-static processing using brief site occupation times. A complete discussion of this web-based service and its options and variations will be provided.