New Blender's event system is working like a charm, with realtime updates, and non-blocking tools.
Check this video to see how I can playback an animation, while modeling, tweaking the interface, materials, rendering, everything realtime.
Blender 2.5 is a development version, try it yourself by getting special builds compiled by the community at GraphicAll.org
Blender is an Open Source 3D Suite.
Check blender.org (the latest official release is 2.49, bit different than the version shown this video)