Wilkin Ng implemented several additions to WinBoard, including a history window, blunder check, and coach mode. They look interesting and useful, but unfortunately I didn't find time to try any of them. XBoard already has a history window, but hopefully the backend portion of the other changes would port over if someone took the time to code the frontend (GUI) portion. If you're interested it this, it's probably best to contact Wilkin to see if he has done more on it since the last time he wrote me, but as a preview I've saved the mail and code he sent me here. You can find the last email address I have for him in the messages.
The changes were made in late 2001/early 2002, so I think they are based on WinBoard 4.2.3. WinBoard 4.2.4 and 4.2.5 came out in December 2001, and 4.2.6 came out in February 2002. Those were all minor bugfix releases, so they shouldn't be hard to merge in with this code.