UPX - The Ultimate Packer for eXecutables

UPX is an advanced file compressor which reduces EXEs and DLLs to around 50%-70% of their original size with little in the way of overhead during runtime. Its distributed freely under GPL but anything you compress with it is excepted and can be freely distributed. For example, I compacted MSVBVM60.DLL down to around 745k and haven't experienced any problems with it.