There is a Mac OS X native version of Simple Text still available. It comes with Xcode as a demonstration of the basic aqua interface. Install Xcode and check out /Developer/Applications/Utilities/Built Examples/
Simple Text demonstrates basic Carbon elements in an OS X context. It shows developers how a carbon app can be written to make use of OS X services, like spellcheck and speech.
Now you can enjoy SimpleText in its full, low resource utilisation glory in a native OS X environment