Syndicate Voidtech. You can summon a copy of any unit on the battle field. Say an enemy comes at you with the Celestian T4, summon a Quantum Echo of it. Getting someones evil twin to kick their arse is fun.
This allows you to play with units you normally can't get, like other secret tech's units, even if its just for one battle.
Echowalkers summon a copy of themselves with all their mods.
A hero with the refresh ability mod might enable them to summon two Quantum Clones of themselvesthemselves, I haven't tested it yet , but I plan too.
Plus techniquely the Zeniths healing ability is a summons (its summoning fresh troops, which is why they are now at full health).
You can also summon Sentries, which is a cool flying arc damage robot drone thing from the Syndicate.
Oh and using that strategic operation that grants copies of I think four defenders is really cool as well.
And Subjectators don't summon, but they do convert enemy units into Indentured.