Thanks! I took a look at the Surf Platform Developers Guide, and I'm sure an experienced developer would find it great, but for someone who is just looking for a way to customize a few elements in Alfresco, it's way to complex.
So, I decided to figure out how to do it:
The elements in the Site Type pulldown box on the Create Site page is generated by an array called in create-site.get.js (which you'll find in the share/WEB-INF/classes/alfresco/site-webscripts/org/alfresco/modules directory). The array definition looks like this:
var sitePresets = [{id: "site-dashboard", name: msg.get("title.collaborationSite")}]
The id refers to a site definition in an XML file, and title.collaborationSite refers to the title text defined in create-site.get.properties. Say you want to add a new Site Type called "Project Site". Change the definition of sitePresets to this:
var sitePresets = [{id: "site-dashboard", name: msg.get("title.collaborationSite")},{id: "project-site-dashboard", name: msg.get("title.projectSite")}];
Now edit create-site.get.properties (found in the same directory) and add a line defining the title text for the new Site Type:
title.projectSite=Project Site
Make sure that the line is added to any localized properties files as well.
Finally, you'll have to create the actual site definition. Edit presets.xml (in the share/WEB-INF/classes/alfresco/site-data/presets folder). Copy the entire definition that starts with the line "<preset id="site-dashboard"> and down to the corresponding </preset> tag, and change the id to "project-site-dashboard" (or whatever you want to call your site definition). You can then customize the default site dashboard layout by changing the component definitions.
Restart Alfresco, and your new Site Type will be available in the Create Site dialog.