I just found out about a site that is trying to create a collection of old business plans. It is called (shockingly) businesplanarchive.org.
I have only just gotten a chance to read over a few of them, but this site is outstanding. For me the hardest part of writing business plans is just deciding what format to use. In the past, I have found reading over 10 or 20 samples to give me a much better grounding to start from. Now, as soon as I find my Million Dollar Idea, I've got a resource of failed companies to guide me. (irony?)
The other cool thing about this site is that it includes a plethora of complimentary material (powerpoint presentations, screenshots, news clippings, etc) that give the business plan context. The site is centered around the dot-com boom and bust.
If you're an entrepreneur, or want to be one some day, should be a good tool.