Linux w/OSX w/o partitioning and wiping the HD?

I don't want to run from a live disk, I want to truly install. But I don't want to partition my hard drive, since I don't particularly relish the thought of starting over. Partitioning seems to be across-the-board destructive, as in get-thy-data-outta-the-way-first destructive.

I'm starting with X11 and BSD here; they're content without their own, quirky partitions. They just want a nice, quiet roommate who doesn't insist on his own wing of the house. :D

Any distros that can just be installed, as is?