QGIS (Quantum GIS) is a sister project to GRASS. Our two projects share programmers, infrastructure, and code when we can.
It features an easy to use GUI interface which may be familiar to users of other common GIS software. Whereas GRASS focuses more on on data processing and analysis, QGIS focuses on cartography and map making. They compliment each other well and you can even use the QGIS GRASS Toolbox to load in GRASS data directly, and run GRASS processing commands from the QGIS menus.
We are both founding members of the Open Source Geospatial Foundation (OSGeo)