I wrote up a summoner PrC for a player a while back that never got used. It seems balanced at first glance but hasn't been through any serious playtesting yet so YMMV.
It's very focused and there's a trade in total power (-2 caster levels) for summoning-specific abilities.
Grand Summoner
pre-reqs: spellcraft:10, Knowledge (planes):5, augment summoning, 3 summon spells (1 of which must be 3rd level or above), a Protection from X or Magic Circle spell. May not be a wizard specialist with conjuration as opposition school.
BAB: poor (+1/2)
Saves: Will good
weapon/armor profs: none
Skills: 2/level Spellcraft, Sense Motive, Bluff, Knowledge(planes, arcana, nature), speak language, diplomacy, intimidate
Spells: caster level improves +1/level except at 2nd and 7th level
Abilities:
1st Specialist, familiar
2nd Faster Summoning
3rd expanded share spell
4th Expert Summoning: combat
5th enhanced familiar
6th Expert Summoning: resilience
7th Expert Summoning: banishing
8th
9th Faster Summoning
10th Horde
Specialist: caster gains 1 spell of each level per day for use with Summon spell. (Does not stack with wizard specialist)
familiar: character gains an additional familiar. Summoner levels stack with other classes that provide familiars for total familiar bonuses.
Faster Summoning: at 2nd level summon spells no longer take 1 full round to cast. At 2nd level it requires a standard action + a move action to cast while at 9th level they require only a standard action.
expanded share spell: spells shared between caster and familiar function out to a range of 1 mile.
Expert Summoning. The summoner has a competence bonus of Summoner Level/2 that applies to various functions.
combat: replace the +1 provided by "augment summons" by the Expert Summon bonus
resilience: Add expert summons bonus to caster level and creature's SR (if any) to resist banishment/dispelling, penetrate magic circles, or defeat SR.
banishment: Add expert Summons bonus to caster checks to banish or Dispel summoned or extraplanar beings.
enhanced familiar: familiar(s) gains outsider template appropriate to character alignment (celestial for good, fiendish for evil, summoner choice for neutral)
Horde: 3/day the summoner may maximize a summon spell without affecting spell level or casting time (as the Sudden Maximize feat)