The one thing that every organism is expected to do is try and reproduce. When you see an organism give its life up so someone else might, it begs the question, why? Social animals, particularly the uber-social ones like bees and ants, have been the subject of the much research for this reason.
There is yet another organism that drawn attention for this reason. Soil myxamoebae are not too charismatic. The fruiting bodies (that you see in the pictures) that they form in one phase of their life-cycle look much like fungal sporangia. But they're quite different. A single fruiting body is made of many individual soil myxamoebae which have aggregated together. The stalk is made of dead amoebae and the 'fruit' on the top are living ones. The living ones will become spores and be dispersed and so the dead have died for anothers cause. A recent paper suggests that in some slime moulds there might be some interesting kin selection going on. Carl Zimmer the author of At the water's edges blogs here and has written a very nice post about them.
The other interesting angle to Dictyostelium discoidieum is that the individuals here are single celled organisms. And they come together to form multicellular super-organisms! Some believe that they may hold some of the clues to the evolution of multicellularity.
The pictures were taken in this lab which works on them in IISc.