There's about a billion and one uses for summouned monsters... lets just touch on the basics... lets use Summon Monster III, the one I used most offen on low levels. 5 round duration at level 3, when you get the spell:
We assume everything can fight.
Celestial bear - move/lift objects, carry you over burning groud (it has resist elements), shield against missile weapons
Celestial bison - Set off traps in long hallway, battering ram
Triton - retrieve submerged objects, short-duration aquan translator, help swim across water without the skill.
Celestial dire badger - Badgers? We don't need no steenking badgers!

Short-duration excavation. Possibly herd larger animals, badgers are crazy
Azer - start a controlled fire, short-duration fire-talking person (forget the language)
Elemental - A lot should be self-explanitory
Thoqqua - One of my favorites... used him to get through bars/walls, burned right through them.
Fiendish dire weasel - Set off traps, carry stuff across acid pools (fiendish template), scale walls
Fiendish gorilla - Scale walls on assault
Fiendish snake, constrictor - Interrogation, take items into narrow spaces
Fiendish boar - good forest distraction, set off traps
Fiendish dire bat - short-duration air escape (for small creaures), retrieve/deliver items in high/low places.
Fiendish lizard, giant - same as weasel, but slower
Salamander, Small - same as Azer, different alignment
Fiendish shark, Large - scare tactic (most things in water fear sharks), help swim across water, attack boats (sharks can makes holes in wood boats)
Fiendish viper - milk for poison (the poison remains when the creature leaves, else it's attack is pretty much worthless
Fiendish crocodile - Help crossing swampy areas
Dretch - Same a lemure
Fiendish leopard - short-duration tracking, catch dinner, help scaling walls
Fiendish wolverine - short duration fighting whillwind

, burrowing