Terry Pratchett is one of my favorite authors, and it's a joy to listen to him speak (although as an American I had to get used to his accent).
If anyone hasn't heard of him, he's the creator of the Discworld series, with various leading characters such as Rincewind the Wizard (the one character I don't like), Sam Vimes of the City Watch, and the witches Nanny Ogg, Granny Weatherwax and Magrat to name a few.
Convention goers have uploaded various talks and such at YouTube, I provide only a couple here:
Pratchett on Writing, at a convention in Germany:
Also from Germany, a somewhat risque joke:
Pratchett in Russia (bad sound)