Seeking tools for uploading GIS data to database?

To import shapefiles into PostGIS, you can use QGIS PostGIS Manager Plugin. It's a GUI for for shp2pgsql command line function. If you want to load KML files, you'd first have to convert those to shapefiles. This is simple: Just open the KML file in QGIS and save it again as shapefile. If you have multiple KML files, you might prefer using OGR Converter plugin to convert whole folders at once.

QGIS and shp2pgsql (and they are FLOSS) are probably most popular for Postgres/PostGIS database as underdark already said. There are probably some FLOSS tools for MySQL and Oracle but i'm not familiar with those. But if you have access to some commercial tools such as AutoCAD Map (via FDO) or Safe FME, you can import any spatial data to almost any spatial database. Probably Manifold can do it also.

Also there is FDO Toolbox which can copy data and its properties from one source (shp for example) to another (MySQL, PostGIS, MSSQL etc.).

As others have mentioned, there are many tools avaliable to perform these actions, from dedicated x2y style command-line utilities through high-end Spatial ETL tools.

I personally use, and love the power of FME by Safe Software. It has paid for itself many times over in increased productivity. If proprietary/un-free software is an option for you, it's worth checking out.

For open source GUI tools, I also have had good experiences using fdo2fdo by SL-King in a production task-specific scenario, and Jackie Ng's FDO Toolbox for quick translation between formats.

Talend's SpatialDataIntegrator and GeoKettle also both boast some impressive ETL capabilities with GUI designers, but I've never had the patience to get through their learning curves. Probably worth investigating if you need full ETL capabilities.