Several people have posted videos on Youtube about Nethack. As of January 2010, there are well over 700. There are musical videos, parodies and other fan material as well as lots of gameplay excerpts.
A few people have diligently chronicled their repeated attempts to ascend with the Amulet. The some main series include...
The Nethack Diaries[]
The Nethack diaries follow an unspoiled Finnish player trying to beat his best level in the Dungeons of Doom and covers 24 videos.
Let's play Nethack![]
Let's play Nethack is a more protracted series through the Dungeons from a more knowledgeable player, who tends to get unstuck in the Gnomish Mines. It covers 152 videos.
Time Lapse Ascensions[]
There are also several 8-10 minute videos showing a sped-up ascension all the way through which are perhaps more instructional.
Hack Slash![]
An ongoing Let's Play series by the YouTuber Dewaldo. The series is newbie friendly and fairly strategy/spoiler-focused. Playthroughs so far include a successful valkyrie ascension and several wizard, barbarian and priest attempts.