Something I've been struck by as I went through this is how difficult it has been to find samples of most of the fan artists' work on the Internet. Even with the recent ones I have had trouble finding anything beyond a random sampling, the occasional fanzine cover, and a mention as a con attendee.
Sue Mason
2003 and 2005 Hugo Winner for Best Fan Artist

Frank Wu
2004, 2006, and 2007 Hugo Winner for Best Fan Artist

There is a very extensive gallery of Wu's work online. In addition to that there is, unlike the vast majority of fan artist, a book of his art available.