Ok you can create it from scratch with intermediate skills on server side languages (PHP, ASP, JSP...), but if you're short of time/money I'd suggest using an image-gallery system, and post your layouts as images, and also add descriptions or attributes to each of these images (which would be the instructions to install the layouts).
These galleries would also be grouped in albums (which will represent your layout categories).
Also you should apply some skins to hide the fact that it's a gallery and disguise it as a template management system
Some samples, free to download:
http://gallery.menalto.com/
http://www.gallarific.com/demo/index.php
http://www.tinywebgallery.com/en/demos.php
Many more at PHP Resource index:
http://php.resourceindex.com/Complet...mage_Indexing/