Celestial Summoning Specialist( Planar Handbook, p. 38)
You can select from a larger number of options when summoning good creatures.
Add one good-aligned creature to the list of creatures for each summon monster spell that you can cast. (Work with the DM to select creatures appropriate to the spell's list of summonable creatures.) Each time you gain the ability to cast a new summon monster spell, you may add one good-aligned creature to the list.
Without this feat, adding a creature to your summoning list requires you to remove one that is already on the list.