Often needed when creating stuff for games:
Blender 2.5x addon, reduce to N bone weights per vertex and/or normalize, optionally remove too small weights, optionally keeping at least one:
http://f00bar-datadump.googlecode.com/files/mesh_dw_4_bone_weights.py
Only works on selected objects; and not just on selected vertices (maybe should remove from toolbar if in edit mode such that user cannot think it operates on selected vertices)