I'm looking for a specific 40k greentext I read on /tg/ way back that I remember seeing a couple of screencaps of.
The story was about kids playing on a fallen titan that'd been overgrown with moss and stuff. The titan reawakens, sees the kids around it, and concludes that the fighting it did in the great crusade has finally brought about the peace it wanted. It then deactivates forever, content in the belief that it has done it's job.
Other than this, any other greentexts of 40k that unexpectedly turn out to be noblebright or heartwarming are very welcome. The longer form stories are pretty well documented over on 1d4chan wiki, so just a quick mention and a link are probably enough there.
image source via google: http://cdn.head-fi.org/3/3e/3e9ee7d0_PCG220_feat_dmo_dmoss4_.jpg