The above link will let you download a special version of Global Mapper which will easily open the SDTS.TAR.GZ files. Once registered, I think it will let you save them as shapefiles as well.
I'm not sure about that last statement because I just downloaded and registered it and haven't yet received my User Name and Registration Key. Maybe somebody else in Cartotalk has done this and knows.
Okay. Exploring the topic a bit further I found the following description of the software:
"The old USGS geodata viewer dlgv32 was the code foundation for the excellent Global Mapper data viewer and format converter. Global Mapper has granted the USGS a license to continue to release a feature-limited version of Global Mapper under the dlgv32 Pro name, primarily as a data viewer.The latest version of dlgv32 Pro was released a few weeks ago, and includes both 32-bit and 64-bit versions.
"The main limitations of dlgv32 are the inability to export data, and the limitation of four raster data files loaded at one time; you can get around this latter limitation by putting all the raster files into a single directory, and then using the File => Open All Files in a Directory Tree option. It has import support for virtually any vector and raster data format you can think of, as well as most projections/datums. There’s also access to online raster data sources, including high-resolution Digital Globe imagery (watermarked, with a daily quota), along with USGS topo maps and black-and-white ortho imagery. Additional features include GPS support, measurement tools, and data attribute query tools. The lack of any export capabilities (except perhaps by doing a screen capture), as well as no way to save a workspace, are big minuses, but it’s still worth a look."
This means DLGv32 Pro won't let you save the data, but it does a very good job of letting you view it. In my opinion, if you've got $300 or so to spare, Global Mapper is a good investment.