In-browser topic modeling by David Mimno.
From the post:
Many people have found topic modeling a useful (and fun!) way to explore large text collections. Unfortunately, running your own models usually requires installing statistical tools like R or Mallet. The goals of this project are to (a) make running topic models easy for anyone with a modern web browser, (b) explore the limits of statistical computing in Javascript and (c) allow tighter integration between models and web-based visualizations.
About as easy an introduction/exploration as I can imagine.
Enjoy!