Minion comes to mind as a good shot - you can nab the +1 coin and still go for four cards (attacking your opponent).
Golem - in long games cantrips can work as psuedo villages with golem. Golem -> cantrip/cantrip is +2 coin, +2 cards, and +2 actions so you can keep chaining golems. Works particularly well when you want chain a lot of strong terminals (e.g. goons, council room/miltia, torturer, etc.). Doesn't work so well in short games where golem is too slow, but very strong in long bouts.
Ironworks - presumably if iron works are worth getting there is some setup that either already likes cantrip or would benefit from IW not being a dead card. Using IW to gain a cantrip is better than a dead IW late game.
Limited draw/action-cash decks. Take library/festival with some trashing. Cantrip gives you coin without decreasing the effectiveness of multiple library plays. Treasures slow down the combo, terminal silvers mean you need more +actions to make the library draws work, terminal draw is counterproductive and non-terminal draw competes on price with the main engine components; as long as cantrip didn't displace an engine component buy, I'd take it every time in a festival/library deck.
Foreknowledge draw. A cheap enough cantrip can allow you to use it to get power cards revealed by look ahead cards (e.g. apothecary, scout, cartographer, navigator, etc.) and play those. The +action/+card look ahead cards allow you to use cantrip to increase the search space of the next card (e.g. playing cartographer - revealing two golds, discarding 2 coppers allows you to use cantrip to draw the copper and for your next apothecary to search through 4 cards instead of 3 for stuff to discard). For setups without +actions/+cards, cantrip can let you nab the one out of 4 top cards that you need (e.g. navigator reveals golem, copper, copper, estate).
Tribute defense. Generally, actions are weaker than coin or draw in tribute games and cantrip decreases the odds of tribute being more than a festival or slightly activated city.
Ultra-lean decks. E.g. You are setting up a KC/Masq pin in a mirror match. Having cantrip still allows you to play the pin (play any number of cantrips then KC -> cantrip -> militia -> KC -> masq), but also allows you to give away a cantrip to a masq without destroying the pin. Likewise, say you build a golden deck of gold/province/bishop/cantrips. Even if you get hit by militia, you can still bishop away a province and churn onward. Ultra-thin Great market decks likewise benefit from adding cheap cantrips.
