UI-View32 is a 32-bit Windows APRS client. The author, Roger Barker G4IDE became a silent key in September 2004, but UI-View32 is still one of the the most popular APRS clients. There are many "add-ons" and more get created from time to time. The official UI-View website is now at http://www.ui-view.org.
UI-View32 is no longer maintained due to the source code being destroyed after Roger's death per his wishes. That said, there is still a very active support group and also a development community creating add-on software for UI-View32.
More information can be found here M0CYP UI-View Web Resource .