I want to display a tooltip when the mouse hovers over a link in my custom rich edit control. Consider the following text:
We all sleep
This is not elegant, but you might be able to use the RichTextBox.GetCharIndexFromPosition method to return to you the index of the character that the mouse is currently over, and then use that index to figure out if it's over a link, hotspot, or any other special area. If it is, show your tooltip (and you'd probably want to pass the mouse coordinates into the tooltip's Show method, instead of just passing in the textbox, so that the tooltip can be positioned next to the link).
Example here: http://msdn.microsoft.com/en-us/library/system.windows.forms.richtextbox.getcharindexfromposition(VS.80).aspx