If you have any of your own "let's make the GM cry" interpretations of spells, why not share? I am positive that many of these have been thought of before, as they seem kind of obvious, but in the hope of others being encouraged to greater originality, here is my guide to the more original uses of spells.
Turn to goo
leaves behind cyberware, and other non living materials, right? Sure you could use it to seriously screw over the cyberzombie comin at ya, but good luck beating their body... So what's it really for?
Why not use it to get rid of implanted RFID tags on the wussy scientist you are busy extracting... You don't even need to damage/deactivate the tags. A braver than average corp lackey swallowed the keys to the safe? Your drug mule got heat on his tail and you don't want to leave surgery scars? Turn to goo, your best and only non-surgical bullet removal system!
Physical mask
Does not say that the target has to be metahuman, or even living. If you beat the OR threshold nonliving systems cannot detect the change.
Oh did I mention cyberscanners are non living, so are MAD's, and chemsniffers. "Nooo, this ain't 12 pound of C12 in this nice moss-lined astral LOS blocking suitcase, it's a birthday cake for me dear gran."
Sure you can mimic the corp you are attacking but when your RFID tags don't match the enemy's IFF has you painted. Why not switch faces with the troll armour bunny, so the mage casts his mana spells at you and all the sniper bullets go flying at your tank?
Another thing you may not have thought of is that a lot of spirits are vaguely your shape, trade faces with your spirit of man - most corp secs are trained not to bother shooting at spirits, and to go for the mage...
Here's another... physical mask does not have to be placed on a willing target. If you can sneak up behind Johny Corp, touch him, and get 4+ successes, all his lovely kit will suddenly be convinced that some bugger with all the wrong biometrics has stolen it. This can be particularly funny in a building that packs auto turrets.
Sure it is a waste of an opportunity, but boy is it comedy when all his safeties lock on and his comlink won't even let him radio for back up. Though it will probably ring an alarm or two.. so all his mates will come and slot him for kidnapping their good mate Johnny.
Laugh as he tries to tell them in his thick Russian accent that he has no idea why he grew a moustash, a mohawk, and a slogan on his shirt saying "Зарежьте корпоративные свиней!" (thanks, Babel, hope I got that near right).
If you are feeling especially munchkin ask the GM if you can change red shirts to green shirts, and Hugo Boss to Eau de sewage overflow why can't you change "Why are you looking at me funny, guys?" to "Ha ha haaa! I Killed Johnny, what are you going to do about it corporate-whore pig-dogs?"
Final trick with physical mask, few mundanes are trained to shoot somebody who is dead already, a few bloody holes in the chest armor, no pulse (Affects multiple senses remember?) and a rapidly cooling corpse can get you a nice cool ride to the morgue. Use at your own risk, sometimes Joe Sixpack likes to make sure... By the way, biomonitors are non-living.
Stealth
Silence on a door means it don't matter if it bangs against the wall after being kicked in. It doesn't even cause the lock it's tearing open to make a sound. Silence on your victim is on occasion handier than silence on you. New meaning to "nobody can hear you scream."
Bung it on a helicopter. That and improved invis removes a lot of the stress from finding an entry point for your run.
Mob mind
A force 5 mob mind spell will take out a group of aggressors in close proximity, imo, more reliably than a combat spell, for less drain and with far less worries about collateral damage.
Mob puppetry allows free action for people not being commanded, but mob mind makes no mention of this, therefore as soon as the spell takes effect you can't do a thing the caster don't want you to. He has complete mental control.
Enemies roll willpower 6 if gm is being silly VS my spell pool of 9 if I am not even munchkining it. While you are practically forced to overcast kill spells to be sure of dropping somebody with them, you don't even have to worry much about threshold on this one. First command, mass broadcast to the mob, kill that enemy who managed to resist this spell.... and if the AOE caught a few of my chummers, so what?
Interesting thing is that from the description of AOE spells anybody leaving the area leaves the effect, anybody entering it enters the effect. You then have 5 rounds of shuffling chummers out of the AOE, then deciding how many mini grenades to feed the rest of them.
Note that mob mind uses the term "mental commands" and does not mention that the targets have to be human. Ever wonder what would happen if a mob-minded flock of 30-odd feral pidgeons was sicced on you? or a Africanised honeybee nest? Or a bucket of black mambas from that exotic pet shop you know? It's not even like if they managed to break out of the spell after x rounds of building up resistence successes that they would come after you, what with this screaming stampy fellow right next to them and all...
You can cast mob mind at a choke point and so long as you are sustaining it all the patrolling guards going through that door have a sudden urge to drink the tasty looking gator aid then take a nap just out of sight of the door.
This kind of spell is ideal for setting up ambushes as half the poor buggers through the door are surrendering before they even know they are under attack, and if you are a quick enough thinker you can get them to call all their mates into the trap for you, and the people who did resist do not know what is wrong until their pals simultaneously arm all the grenades they are carrying.

Mob mind and quickening make things like that even more intersting. You enter my house and you my bitch even as you are kicking the door in.

Remeber also that anything includes getting their security rigger to hand control and protocols of the building and drones in it over to your rigger, the guards to transfer all of their cash into certified credsticks, and everybody to remove and disable their commlinks then say out loud a list of the cyber they have implanted and abilities they have. If you managed to catch a mage with this (lucky lucky) get them to dismiss all of their spirits, and hand over their foci and fetishes.
So what if it wears off after a few rounds? by that point they have had time to give you all of their cash, disarm themselves, and kneel patiently for a coup de grace from your street sam.