I've used a number of references in identifying the different models available. When you're scouring ebay or walking around in flea markets looking for good finds, the model name is not always known or given. I've found used these websites to help me identify each one of them and sometimes be able to date them as well.
1. Victorinox Tool Finder - This is the official tool finder you'll find in the Victorinox website. It's a tool builder of sorts with drag and drop function that will help users find out which model to choose given the tools they want. Mostly caters to buyers of current production models.
2. Swiss Army Knife Selector - a comprehensive and interactive way to identify the model name of the swiss knife based on the tools, overall size (length, thickness, number of layers), material (Alox, Cellidor, Nylon, etc). This tool covers both current production and out-of-production models. Check it out at SAK Selector.
3. SAK Wiki - This is a great website for SAK collectors, period! You'll find great and comprehensive information on most of the Victorinox and Wenger Swiss Army Knives here. It's easy to use and a very good website to visit to familiarize yourself in the various models and see if they are discontinued, rare, vintage, collectible, etc.
There are other websites that you can visit to find great information and history behind each model. Use google to find SAK Collector websites. I also frequent www.multitool.org which is the go to forum site for all pocket knives and multi tools. Swiss Army Knights is also a one-stop shop for all Swiss Army Knife information. It has a link to the SAK Selector, SAK Wiki and other forum/trader sites right in their website. You can read interesting articles as well and will further your knowledge about the SAKS we love to collect.
If you have other good websites you want to share, please write them in the comments section. Thanks!