Musings on an Interstellar Ark
Interstellar Ark
One of the most interesting/fun things I've read on the interwebs. Some interesting ideas in there, and at times the way the post is written is poetic. Skip towards the end if you're not a physics nerd (or aren't at least interested in the physics side of things).
PS: Be sure to read the comments too, a lot of good discussion between people and the author.