Recently I added Sean McCullough's Javascript implementation of the Perlin noise algorithms to jsGameSoup. These Academy Award winning algorithms by Ken Perlin are fantastic for adding procedural content to your games.
The RPG map above with grass, water, road, sand, and tree tiles represented in the different colours, was produced with the simplex noise demo here. It can randomly generate a basically infinite number of unique RPG maps of basically infinite size. Check the jsGameSoup documentation for more details.