Just a few ideas for those wishing to use summoning spells in a game/campaign but are too scared because of having to scan through endless pages of bestiary's and monster manuals for suitable monsters.
This ruleset does require a bit of work on the DM's part in the short term but then should make life easier and more rewarding in the long term. The ruleset could even be applied to the polymorph spells as well as summoning spells.
First assume that a character's knowledge of creatures is not infinite and so will only know how to summon a few favourite creatures that he maybe has contacted or formed a relationship or that is a gift from his god or the god of magic (like a personal servant/bodyguard).
In order to emulate this each character should have a number of creature slots equal to his Int/Wis/Cha bonus (players choice) +1 for every summon monster or summon nature's ally spell he possesses.
Then the DM can allow every monster in the bestiary to be summoned or just a few select ones and assign it to a summon spell based on its CR (1-2 for summon monster/nature's ally I, 3-4 for summon monster/nature's ally II, etc).
Then when a player learns a new summon monster spell he can assign a creature slot to one of the available creatures for that spell. Every time he casts that spell he summons from the creatures he has in his creature slots.
In order to represent the different summon spells you could allow summon monster to only summon Animals, Magical Beasts, Outsiders, Vermin. While Summon Nature's Ally could be used to summon Fey, Animals, Vermin and creatures with the Elemental subtype.
As a DM you can then give the PC a copy of his creature's stats to use in encounters that it is summoned.
If you find that the creatures are too hard and unbalance combat then creating a simple Minion template (-2 to all stats and 1/2 hp) to be applied to all summoned creatures would limit their power and prevent unbalancing. For progression for players in summoning you could allow a feat or class ability remove this template, or even add an Elite template (+2 to all stat and full hp), as well as a feat to add more creature slots (3 per feat)
You could allow templates to be added to summoned creatures (templates take up a creature slot as well).
You could even allow if you wish for the players summoned creatures to gain xp and be improved upon (assuming the creature progression allows it) and at such a time as the PCs are able to call their creatures rather than summon, they could even provide them with magic items.
You could even design a summon minor monster or natures ally spell as an orison/cantrip with infinite casts but only allow creatures with a CR of less than 1 which will allow creatures like a cat or rat to be summoned.
If used correctly players should become attached to their summoned creatures and you can use a scroll of summon monster with a new exotic creature on as a new piece of treasure (they would have to learn the creature with a suitable knowledge arcana roll), or personalise your player nemesis' with particular summoned creatures themselves.
I think the same style rules could be expanded to a new set of polymorph spells or illusion spells; one for each spell level allowing the caster to change into or make appear ever more powerful monsters. And if you allow summon undead spells you could easily apply this ruleset to a set of summon monster I-IX spells
Hope this gives people a few ideas on expanding a simple and underused but often abused type of magic
