The basic weaving concept behind the three-strand braid can be applied to any number of strands. Flat braids with an even number of strands will have asymmetrical tension on the sides, which gives it a natural twist when the braid is hanging loose. Braids with an odd number of strands will look more like a standard weave.
I like to do 5- and 7-strand braids most often (beyond that I run out of fingers), and I cross the strands such that the hair is added underneath, such as in the dutch braid.
For instructions on how to do a 5-strand braid, see the DreamweaverBraiding site here.
For more pictures of multi-strand flat braids I've done, see the Flickr set here.