Seeking open source GIS platform?

Here's a vote for Quantum GIS with GRASS plugin enabled as your desktop application. (QGIS is available on Windows, MacOS X and Linux). OGR/GDAL will take care of nearly any file format. Store all your data on PostgreSQL/ PostGIS and serve it out with Geoserver.

Link to QGIS API Documentation.

UDig is Java based so it will run on all the major operating systems with no issues. Has good editing support.