Depends on how low-level you're talking about, and how many you have. The drakes are alright, especially with dragon master to improve gem efficiency, ferex. Cave drakes especially can be an early answer to a lot of some low-prot or low health nation's problems, and the fire and ice ones can add a nice lil'bite on the cheap that can still take a lot more of a hit than most recruitable troops. With dragon master, you're just getting a ruddy nice HP to gem ratio, if you just need something to arrow catch.
Just looking at level 3 or below in the schools (this is in CBM 1.6!), lemme' see if I can remember the notables...
In conj: Black servants are apparently alright mini-thug/raiders -- built in life drain and ethereal. Give 'em some armor (5 gem black steel and horror helm, maybe) and they'd do alright against lighter PD and indies. There's the drakes, as above, and sea serpents, which can help out most of the underwater nations in their indie-squishing -- as with the drakes, they're a lot of HP for a fairly low cost. Summon animals can be pretty interesting -- a lot of the units it pulls up is better than most plain human troops, and some (Moose!) are pretty beefy, which makes it a pretty good spell if you need something quick to catch arrows or blunt a cav charge or something. The awaken sleeper spell is a go-to for low-tier thugs, and banes are a 4 gem bane blade that walks around.
In construction, you've got clockwork horrors and corpsemen as summoned units. The clockworks are good for chaff, especially if you're someone without general amphibian access. They're especially notable when going up against large (size 4+ non-tramplers) enemies, as they're fairly small and have two attacks, so they're excellent at stripping defense away. Corpsemen aren't just souless -- they've got more hp and lightning immunity, and they're cheap as all get out. Not much able to summon them natively, but they're alright have some low-level mage you've got puttering around call up in between battles or shuttling troops. Extra chaff, if nothing else. E: Well and manikins and wooden constructs. The manikins might be interesting to throw at high encumbrance enemies, but they're otherwise mostly worthless as anything (and even, to a degree) except chaff. Wooden constructs are, again, mostly worthless. About the only thing I could see using them for is to distract the AI from actually useful large monsters, or if for some strange reason you really needed some extra no-morale HP somewhere in your lineup.
Enchantment has reanimation as about it for mass-summoning, and it's pretty worthless. Even at two gems, 10 longdead are kind a ripoff. But! Ench also has mound kings, which are basically the indie-commander of undead, and can lead things underwater if you need that sort of thing, and revenants, which are 9 gem dark knowledge casters, something your better death mages call up to free them up to do other things. They're also alright for chasing armies around spewing frighten at th'enemy. I also hear good things about watchers, but I've never personally used them. They've done alright when I've ran into them, though -- very tough, incredibly high precision lighting blasts, excellent patrollers if you can spare something to physically patrol a province.
And then there's blood magic, of course. Spine devils are one of the heaviest infantry units in the game, you've got bind devil and frost fiend, both solid for the price, and cross breeding, which can both produce a lot of chaff and some fairly impressive larger critters. Hella'good if you're running a luck 3 nation.
It's usually not a good idea to run a totally summoned army in the early game, though, for most nations. Summons are generally supposed to be force multipliers or used to plug holes in your army's TO&E, like no amph access or heavy inf/arrowcatchers, etc. Later, you've got the mage and gem output to really pull full armies out of the air, and they can be serious additions to your main combat line or raiding potential, especially in support of SCs, either directly in combat or by threatening the surrounding area while the big boys do the real damage. Early, though, they tend to be just garnish for your recruitable armies.