It depends on what you plan to do. TatukGIS DK is a very good GIS library. We use it since 20 years. It offers a great map viewer component and the enterprise edition covers all common spatial formats. It's not expensive compared to ESRI products, but it's not cheap either.
If you just have some spatial data in a PostgreSQL/PostGIS database you can build your own SQL statements quite easy. Geometries are represented as WKT which is quite straight forward to understand. But of course it means programming work on your side.