MRS  Some important characters got stripped out of my previous hint. If you know a little XML or HTML it'll be obvious.

Surrounding "com.adobe.encapsulated-postscript" you will see some XML tags. They need to appear around "com.adobe.postscript" as well. Just duplicate the line and remove "encapsulated-" from the new line.

The original line says

[greater than]string[less than]com.adobe.encapsulated-postscript[greater than][slash]string[less than]

where each item inside [] is replaced by a single punctuation character. The added line is:

[greater than]string[less than]com.adobe.postscript[greater than][slash]string[less than]

Sorry for any confusion.  
(Version 1.0)

MRS  Excellent tool. And a little hack makes it even better! Here's how to make it handle .ps as well as .eps files:

1. Go into /Library/QuickLook.

2. Control-click on EPSQLPlugIn and select "Show Package Contents".

3. Make a copy of Contents/info.plist just in case.

4. In your favorite text editor, find this line:

com.adobe.encapsulated-postscript

5. Add this line beneath it:

com.adobe.postscript

6. Save the file.

7. Reboot.

Voila. Quicklook now shows you .ps files as well! I don't know why they don't deliver it with this functionality already enabled.  
(Version 1.0)

SNOOPY31415  Great applications!

I think it makes quicklook load a bit longer (not much, like 1-2 sec) before displaying stuff but I could be wrong on that.

It doesnt display ps files but you can simply rename them into eps and it will display them.

For example rename "fig.ps" into "fig.eps".

It stange because the only difference is that eps has a bounding box while ps don't. The rest is the same.  
(Version 1.0)

MARCO VIGNATI  Great plugin, thanks! It would be very useful if it could handle ps files too...  
(Version 1.0)