What are some creative ways you or your players have used spells?

Recently, a player was possessed by a ghost. I (Wizard in an OSR) used polymorph other and turned him into a snail and just waited. then I threatened to get a bottle to put it in...and he released the body and we killed the ghost. 

Same OSR wizard but early in campaign (level 1), we entered a tavern and a giant was wedged behind a table and had been for months. he didn't remember how he got there but the tavern was getting tired of feeding him and the giant was taking attribute damage. Told him in Giant (language) that I'd come back for him in 8 hours. Came back with enlarge memorized and reversed it. He had to run up the stairs before it wore off and the ran through the tore (broke it) before it wore off. This was the desert so the environment came in but we wound up with a giant crewmember for a few months till he died). He had to sleep on deck of skyship covered in tarps. The DM had not considered that we could save the giant. 

Lightning lure and thorn whip can be used by flying players (owlin, fairy, etc) ... to fly above a target and cast the spell. If it works, the target gets pulled up, takes damage, then fall damage, then maybe winds up prone. 

We were interrogating someone who kept getting assassinated and I (Druid) kept bringing her back (Revivify) to finish the interrogation. I Also revivified or reincarnated crewmembers (spelljammer this time) to see what was killing them (succubus).

5e 2024 rules I had a halfling druid riding a hyena or vulture familiar. 

As wizard (5E), I used spells to do rogue-like things. I set up some cantrip based illusions with stage props (this was a performance) to make it look like i turned invisible (I did) and was walking into the crowd (I wasn't but they could hear my footsteps (minor illusion) among them) ... so that I could steal something in the storeroom behind the stage.

I've used Create Water to create rain in a dry area to reveal the position of someone in a Cloak of Invisibility. The dry spot showed their current location. The only DM buy-in required was that rain would do that whether magical or not.

Oh, that/s something you do in the game Divinity Original Sin 1 when an invisible enemy attacks. You cast the "rain" spell and you know where he is. they also have a spell that is lie Grease, that works too. 

I've also heard of people throwing sacks of flour around. :)

Launched a fireball up in the air to cast a brief instant of light that revealed an army sneaking toward a bridge. Because of that, the DM had to rewrite some things and a city that was meant to fall was saved.

My party was fighting a giant mechanical sky worm. We had defeated it, but it was about to blow and not dead yet. It charged for our ship, hoping to take us down with it. I was out of spell slots, and had a few sorcery points left. I was playing an aberrant mind sorcerer.

I had my sorcerer fly beneath the sky worm, and use my last few sorcery points to cast dissonant whispers, putting all my remaining resources into the spell to heighten it. The sky worm failed its save, and was forced to fly straight up, where it exploded in the air harmlessly

One possible use of Shapechange is to take the form of having two sets of arms to cast spells faster. A DM may reject this idea. 

Quote from Kurus >>

One possible use of Shapechange is to take the form of having two sets of arms to cast spells faster. A DM may reject this idea. 

 That is not a valid use of the spell.

Quote from Kurus >>

One possible use of Shapechange is to take the form of having two sets of arms to cast spells faster. A DM may reject this idea. 

 A DM would reject this, because a spell that takes an action takes an action, even if you have 1,000 arms.

Yeah, you can certainly get more arms by casting Shapechange, but there's nothing anywhere in the rules that says that having more arms lets you cast spells faster, so that part wouldn't work.

There is even a playable species that already has four arms (the Thri-Kreen from Spelljammer: Adventures in Space) but its features make no mention of those arms allowing them to do anything faster, including spellcasting.

Oh, I derailed a couple of session because we were not supposed to outrun the Eberron apocalypse storm. DM had thought of the spells my druid could use to get us ahead of it...and I didn't have them. So, I summed 4 giant eagles (hello LOTR) and a gryphon (figurine of wondrous power) for players to ride. DM asked for an INT roll...but I reminded him I'm a WIS caster. Then he had me roll WIS to see if I could get the beasts to let us ride them without a saddle (or special saddle), and I passed. I don't see why they could even carry people in their beaks if needed but i had passed the roll, so riding was ok. I also sent players ahead on a living flight spell (hello eberron) and we also bypassed an encounter (skyship being attacked by living fireballs) due to our route and a split party (apparently a VIP died because of this). We had to stop for the evening and come back in 2 weeks because we had bypassed his material.  

That's a case of the GM forgetting that when you need to have the party trigger certain encounters, you have them happen regardless of what route the players travel.  This is especially easy for something like an airship since it can show up wherever you want without much in the way of geographical issues.

Most insane chain of events ever. I cast grease on the floor during a fight with a drow priestess (we were in a chapel). She slid in it, and the DM said she fell prone. Cue the party whaling on her. The next turn, I asked if there was a window in the chapel. Our DM said there was. I cast Defenestration and shoved her out a window. I only meant to send her lower down the chapel. She fell into a bottomless pit. We were supposed to have had a couple more encounters in the city, but we just got out very quickly. She had already used her Levitate spell for the day.

Oh, everything was there: airship in our way, living fireballs attacking it. But, two party members (wizard and rogue) fled the second the storm hit (they used magic to make it to a (different) skyship in the distance). I sent a couple of party members and a prisoner ahead on a living Fly spell...and once I set the destination, it leaves and I have no control over it. That left a few of us (monk, barbarian, druid) on flying steeds and we were not really capable of NOT taking on 3 or 4 (I don't remember) living fireballs alone that were attacking a skyship. We had no idea there was a VIP on board. We just flew higher and over the ruckus. DM should have had the VIP waving from the deck or something...I had an unburdened giant eagle.  Also, the party members who would have wanted to try and save the skyship were not with me...they were the two who fled first. My group just wanted to escape the apocalypse storm.

Years ago (1e) cast wall of ice sideways in the hall of the hill giants then the dwarves in the party ran under the wall slicing femoral arteries wiping out most of the giants.

Back in 2nd Edition, I was in a group when we encountered an Adamantine Golem on the top floor of a castle.  We didn't have any weapons that could harm it, but the party's wizard pulled out a scroll of Disintegrate and cast it on the floor under the golem.  It ended up in the basement.

You might try Rock to Mud then Mud to Rock to try to trap a monster in stone. 

lit a barrel of gun powder so the fighter could cast fireball. 

I’ve used that rock to mud to rock trick in downtime to build castles, bridges and civic buildings.