To the Stars - I found this through HP:MoR's author notes so you may have heard of it already, but its awesomeness makes it worth mentioning anyway. It is an enthralling transhumanist, quasi-rationalist, far future sequel to PMMM. It also bears a strong resemblance to the Old Man's War series in the same way that HP:MoR is clearly inspired by Ender's Game. It (generally) stands on its own, but has major spoilers for PMMM and to a lesser extent the cannon spin-off PMOM.