This was actually originally a part of siyoa's UI mod, but I figured I should make it available here as a stand-alone mod in case anyone wanted it.
It adds a new side panel, the "Population Report" that shows all of the different species on each planet in your empire. The buttons and mini-planets in the side bar can be clicked on to quickly transfer population between planets, so it is a great tool to easily manage population transfers even when you have a very large empire. The population report is accessed by clicking on a new icon in the game's top bar, which also shows the total population of your empire.
EDIT: Sorting now works properly