M-e-h, formerly known as BlackButcher (before he changed his name) is a player widely known for...
The glitch he found called 'Santa Dupe', this allowed him and a few other players to obtain free santa hats (and other rares) from the Grand Exchange and duplicate them. This glitch has been patched by Jagex around July, 2008. Although he abused the glitch, he has not been banned yet, possibly because no-one knew about it at the time it was happening, so they couldn't report him.
Here is a quote from Jagex:
"Update - We have fixed a major bug in the game that allowed players to get items they didn't earn thus gain unfair advantage."
On a video, recorded before Jagex patched the glitch, M-e-h shows how he did it, but the video has been removed by copyright claims of Jagex.
M-e-h has since then changed his name to Me h.