The Malconvoker (Complete Scoundrel) is widely considered the be-all, end-all of summoning PrCs. Extended summon spells for free, and later you get twice the number of evil creatures you summon.
However, this latter point makes it clear that this PrC is intended for use with Summon Monster, not Summon Nature's Ally.
Another option would be Thaumaturgist (DMG), which is also good, but needs the ability to cast Lesser Planar Ally to enter, so is more for Clerics.
Yet another one: Paragnostic Apostle (Complete Champion). Not really summoning-focused, but a 1-level dip can give all your summons Fast Healing for free.
However, what I'd recommend is this: go entirely without PrCs. As long as you have Augment Summoning (Half-Orc Druid sub levels give this for free, otherwise requires mostly worthless Spell Focus: Conjuration) and Greenbound Summoning (Lost Empires of Faerûn), your Druid summons will be very, very powerful - and your other Druid tricks are nothing to sneeze at.
Really, going full Druid with the right feats (heck, even with the wrong feats) makes you a very competent summoner, as well as a generally very versatile and powerful guy to have around!
