Many people underestimate or simply ignore the random and often seemingly useless consumables they find on the occasional PvE vendor. While some are cosmetic or impractical, others can be game breaking if used properly.
I'm quite bored with the game and will give you my list of consumables, where to find them, and how I recommend utilizing them.
Ash Legion Spy Kit: Purchased from Kyra Sharptracker in Plains of Ashford. Let's start with the king of cheap consumables. Pop this, and you instantly vanish for 10 seconds, provided you don't move. However you Can use it while moving and you will still get a few seconds of invisibility. NO cooldown (what?). You can keep popping these to stay invisible as long as your supply lasts. At a cost of 28 karma per pop, they are very affordable. Reviving a fallen ally in a dungeon? Pop one. I use them if I get locked in a knockdown combo by the Husks in Twilight Arbor. They are especially annoying for opponents in WvW. You shouldn't go down in small skirmishes in WvW as long as you have these. The applications of such easily spammable invisibility are many. This combos well with other consumables and I'll touch more on that later. Edit: Does not work in WvW :(
Ogre Pet Whistle: Purchased near Gladefall Run in Iron Marches. This is decent. Gives you a random ranger pet for 5 minutes. Only one at a time but as soon as one goes down you can use another.
Tier 5 Boon of Regeneration: Purchased near Breekeelee POI in Bloodtide Coast from Muirikroomk (underwater). Gives you decent health regen and lasts quite a while, handy to have. Edit: I've been informed that this NPC's inventory has been disabled as of last week.
Experimental Teleportation Gun: Purchased from an Asuran at the Valance Tutory WP, Timberline falls. The portal aspect is nice, but what I really use it for is the rocket jump aspect of the gun. Some classes have a similar ability already but this gives you a secondary way to cover a large gap and allows friends to travel through the portal if they are following you in a jumping puzzle. As a Mesmer I use it when my blink is down to evade enemies. You can use the Ash Legion Spy Kit immediately after you start your jump, to become invisible while you're in the air.
Miner's Explosive Charge: Purchased from Dianna in Kessex Hills. These are lots of fun. Set it, wait 10(feels more like 7) seconds and you get a nice AOE knockdown explosion. You can set these and Remain completely invisible using the Spy Kit. Sneak behind the zerg in WvW and wreak some havoc ;)
Charr Mine: Purchased near Raintimber Mill in Diessa Plateau. Applies a nice burn to enemies that step on these. Very nice in condition damage builds and they're very cheap. Why not set one before a difficult pull in a dungeon?
Seed Turret:Purchased near the Thorp Waypoint in Caledon Forest. Another good one to drop in or before a difficult fight. Gives you 3 different options of turret-one heals, one slows, and the other does damage.
Dam Debris: Pick some up from Foreman Flannum in Queensdale. Single target knockdown ability with no cooldown. Enemies in Dungeons will become immune after the first knockdown. Go piss off people in WvW with them.
Alright those are most of the consumables I use regularly. There are a few others that I didn't mention but these I've found to be the most useful. The process of quickly opening up your inventory and using these can be automated, but you may get banned for doing so. I don't think you'll be particularly high on the "to ban" list, but you never know. I recommend an easily accessible inventory keybind.