I am in the process of creating a topology checking plugin for Qgis. Although Qgis allows you to create valid geometries while digitizing, the checks to ensure correctness of existing data is somewhat limited. Every time I use the excellent topology tools in Esri’s ArcMap( a commercial GIS product), I have imagined ways of implementing this on some open source GIS.
Here are some screens of the current working version. It is in the early stages, so don’t get frustrated with its bugs if you happen to see it in Qgis.