The SDK has a lot of the source for the Widgets. You'll have to dig around to find all of the pieces, but they are there. Look in the "ContentFragments" folders of most areas. For example, I've been poking around the ForumThreadList widget.
From the SDK, you have these files:
CommunityServer.Forums\ContentFragments\ThreadList.cs is a widget that is externally implemented.
The external implementation can be found in CommunityServer.Web/Utility/ContentFragments/Forums/ForumThreadList.ascx
This user control conditionally displays two different user controls: ForumThreadListSimpleView.ascx and ForumThreadListStandardView.ascx in the same directory.
ForumThreadListSimpleView.ascx uses the tag <CSForum:ThreadList> which is implemented as a class defined in CommunityServer.Forums\Controls\Threads\ThreadList.cs
So, as you keep stepping through the different pieces of the SDK, you can get a complete picture of how the Widget is actually implemented.