Download ImgExtractor 1.3.4 by And_PDA
(Program page here
). To use, open a command prompt in the directory where ImgExtractor.exe and system.img are, and enter:
ImgExtractor.exe system.img System -i
where system.img is the file you're extracting and System is the folder it will extract all of the files to (it will create one if it doesn't already exist). The "-i" is optional, but it shows the individual files as they are extracted (verbose output), and is more fun to look at than an idle window.
See the readme or double click on ImgExtractor.exe for more usage info.
EDIT: Download is now linked to my DropBox and added with readme below.