I know I've posted Alice X. Zhang art before—she has a crazy eye for colour, and works in more fandoms than I can even follow (Wong Kar Wai! I adore In The Mood For Love, but I never expected to see fanart of it, much less nestled between Doctor Who and Supernatural).
Anyway, her brilliance goes without saying, or my inarticulate fangirling (I fail at colour, so hard...so I didn't even know how to describe what she's doing, but there's a rainbow in every stroke, and it all works). I was just catching up on her Tumblr and she mentioned she's had work licensed by the BBC! So not only can you buy her stuff through her (she's a professional) but also some pieces from Big Chief Studios Limited.
Fanart has given someone like me whose art history knowledge is rudimentary at best (I'm exploring!) exposure to so many talented professionals and amateurs alike, and there's no question my work benefits from that. This is as heartening as seeing a favourite fanfic writer go pro (looking at Temeraire, among others..).