Still as of Sitefinity 3.7 SP3, it is still really hard to find out all the available WebUIEditors and Selectors available for Sitefinity properties during Custom Control Development.

With some snooping around using .NET Reflector, you could see they are all over the place.  Most of them are undocumented so be careful :)

These are the 20 WebUIEditors and Selectors currently found in the shipping 3.7 SP3:

Telerik.Blogs.WebControls.BlogsSelector
Telerik.Cms.Engine.WebControls.Categories.CategoriesSelector
Telerik.Cms.Engine.WebControls.ContentSelector
Telerik.Cms.Engine.WebControls.HtmlContentEditor
Telerik.Cms.Engine.WebControls.RssContentSelector
Telerik.Cms.Engine.WebControls.Tags.TagSelector
Telerik.Cms.Web.UI.CmsUrlWebEditor
Telerik.Cms.Web.UI.DhlIdEditor
Telerik.FileManager.UrlWebEditor
Telerik.Libraries.WebControls.DocumentsSelector
Telerik.Libraries.WebControls.DownloadListSelector
Telerik.Libraries.WebControls.GallerySelector
Telerik.Libraries.WebControls.ImageSelector
Telerik.Libraries.WebControls.LibrarySelector
Telerik.Lists.WebControls.ListSelector
Telerik.News.WebControls.NewsSelector
Telerik.Notifications.Newsletters.WebControl.GroupSelectionList
Telerik.Polls.WebControls.PollBoxEditor
Telerik.Rss.WebControls.RssFeedSelector
Telerik.Search.WebControls.IndexingServicesPicker

These are the class names with their full namespace that needs to be ...