I refer to this wiki page all the time:
It has a list of all the built-in properties that you probably don't know exist because they're not strongly-typed properties on the page. Rather, they're indexed properties in the "Property" collection.
You have the same overview in the SDK: http://sdk.episerver.com/library/cms6/html/P_EPiServer_Core_PageData_Property.htm
I'm creating a couple of pages in code and would like to set the sort index for these pages. When I try newPage.Property["SortIndex"].Value = 10 I get "...not set to an instance..."-error and I read somewhere that it's actually called PageChildOrder but that didn't do any better. What's it called?