Here are links to various Seismic related tools and software that I have found useful.

They are in no particular order. Most are free at time of writing. Those that are not free provide very good value for the money.

This is the best and most versatile SEGY viewer I have found, and it's free. Current version is 0.2. A very impressive piece of software.
GeoCalc 4.20

This is Coordinate conversion software. It's a little strange to use, but it works well and is free. After installing, click 'Process' then 'Manual Input' That gets you to the page where you can choose Coordinate Systems and enter coordinates. A very handy utility.

This allows you to convert between various different GPS file formats. It does not perform coordinate transforms. It will allow you to convert Garmin to Magellan for example or straight to KML or GPX or a bunch of other formats.
TatukGIS Viewer

From their web page:
"The free TatukGIS Viewer opens most GIS/CAD vector, raster image, and grid file types, most ArcView, ArcExplorer, and MapInfo projects, and projects exported from ArcGIS/ArcMap using the free Arc2TatukGIS plug-in"

For me, it's greatest feature is the ability to join GeoTiff files and save the result as another GeoTiff file, complete with a correct World file.
It also allows you to resample to keep the resulting large file a managable size.
SEGD to SEGY Converter

This is a small program I found on the Web some time ago. I do not remember where, and I don't know who wrote it. If anyone does know its origins, please let me know so I can give credit to the author
3D Field Pro

From the web site:
3DFieldPro is a contouring plotting and 4D-3D data program. It converts your data into contour maps, surface or volume plots. 3DFieldPro allows create the contours (isolines) on surface and isosurfaces in volume from regular data set or scattered points. All aspects of 2D or 3D maps can be customized to produce exactly the presentation you want.

This is a great alternative to Surfer - much easier to use and much much cheaper.
SEGY Detective

A great little utility to view SEGY headers