N1BIOS is a fun little throwback to the days of verbose system startups. It looks really legit and it made everyone I know that is remotely nerdy laugh hysterically. I love it. You, too, can go back in time by downloading it HERE (this one is flashable, so youll install it through recovery).
This one consists of some sort of disembodied mechanical automaton welding the Android logo onto the fabric of space itself. Nice. Get it HERE (non-flashable).
This bootscreen must have been incredibly involved. Wireframe Andy spinning under some system startup text? Yes. Yes, we approve. Get it HERE (non-flashable).