+1 on business model canvass over "business plan." A business plan of the kind I was schooled to write in B school is now pretty widely viewed as an anti-pattern for endeavors with high levels of uncertainty (i.e., startups). I recently made good use of Spark 59s lean canvas tool.
I use LivePlan and really like it. It's pretty flexible, though admittedly
some aspects are geared more toward "physical products" than we might
like...you have to play around with the financial modeling a lot, for
example, because it automatically assumes you have a fixed cost of goods.
In general, I think it's worth the small investment, because it both
disciplines and organizes your thought process about your business, and
produces a really very attractive end product.