The summon monster lists of 3.5e never made much sense to me, since the monsters tended to be of fairly widely varying power levels. The summon monster spells in the Pathfinder RPG seem a little more systematic.
For these generalizations, ignore the CR boost provided by the fiendish/celestial creature template, which occasionally puts creatures outside of this range. Summon monster I generally summons CR 1/3 to 1/2 creatures, with the exception of the CR 1 riding dog. Summon monster II summons CR 1 creatures, with the exception of the less powerful giant centipede, while summon monster III always summons CR 2 creatures.
Summon monster IV summons creatures of CR 3 to 4; summon monster V summons creatures of CR 5 to 6; summon monster VI summons creatures of CR 7 to 8; summon monster VII summons creatures of CR 9 to 10 (mostly 9); summon monster VIII summons CR 11 outsiders; and summon monster IX summons outsiders of CR 13 to 14.
So, for example, when putting together an alternate summoning list, a CR 2 axe beak would be an appropriate creature to summon with summon monster III.