This is TOPCAT, Tool for OPerations on Catalogues And Tables. It is a general purpose viewer and editor for astronomical tabular data developed within the UK Starlink project.
Related software products are
The Starlink project under which TOPCAT and friends have been developed has been shut down as of July 2005. At the time of writing, this means that there is currently no provision for continued support and development of the software. The author is currently pursuing possibilities for further funding, but it's not yet clear whether or how these will work out. Probably at least a minimal level of support will continue to be available, one way or another. The email address above may or may not continue to be active; check the TOPCAT web page for news in the future.