Working Quick Look Plugins For Mountain Lion
If you install Xcode, most source code files are shown (albeit without color) natively. The "without a file extension" makes things interesting since unless the file is typed, quicklook (and the spotlight importer responsible for determining what type of file each one actually is) has less context to decide how to classify a blank README file.

Have a look at the qlcolorcode plug in for colored syntax quick look and QLStephen for README and other ASCII plain text files.