I personally always bring with me Synthesis Scanners, Large Team Heal Restore, Large Squad Energy refills, Ciphers, Medium Squad Ammo refill, Air Support, Archwing Launcher (plains of Eidolon) and Fishing rod (plains of Eidolon) and some Helpers/spectres for good measure :)
This gear/consumable can be obtained from Cephalon Simaris and he can usually be found in Relay stations (its usually quite cheap to get).
Synthesis Scanners can be temporarily equipped via your gear slot to scan various targets or even sometimes see through walls in missions. Storyline is usually hidden in blue-ish light-cubes on mission nodes and can be scanned – sometimes this is required to complete Junctions.
Large Team Heal Restore
This consumable refills yours and your allies life in missions quite effectively and can be obtained from either Steel Meridian or New Loka for 25k standing and requires rank 3 within the syndicate. You craft it 10 at the time and the blueprints stays forever once obtained first time.
Large Team Energy Restore
Same deal as with Large Team Heal Restore but instead of Health being restored, its energy – very useful in lack of a Trinity in the team :)
Medium Team Ammo Restore
Same deal for this as with the two other restores above, except refills ammunition – very useful if you got a lot of fire-rate mods on your weapon and your Ammo mutation mod feels insufficient.
I use Medium of this because I personally have Cephalon Suda, Arbiters of Hexis and Steel Meridian with 100k standing each, and I cannot be arsed to “re-specc” my syndicates just to obtain the blueprint for the Large one since all I ever have needed was the Medium one – which blueprint you can get from the Chem Lab in Clan Dojo. But if you have the time to climb all the way through other syndicates and feel you really badly need the Large one, go for it :D
Air Support charges
Blueprint for these is automatically available in foundry and it adapts with whatever landing craft you are using (mantis, scimitar, liset or Xiphos) – it adapts in the way that it does different things depending on whichever landing craft you are using to missions and these charges send down support of specific kind based on your ship to you in your missions if you have it equipped as gear and use it in a mission.
Quoted from warframe.wikia.com this is what it does for each ship:
Liset: Ordis will Override security, sending a beacon that disables lockdowns, resets alarms, and disables security cameras.
Mantis: Ordis will deploy a Med Tower (which resembles a Life Support Capsule). It will heal the player for 100 health on each activation and may be activated up to 25 times by any player in the squad.
Scimitar: Ordis will launch an Air Strike, sending Carpet Bombs crashing around the player, each dealing 25 damage.
Xiphos: Ordis will launch a sentry turret which fires on nearby hostiles.
In all fairness even though I have a few of these always equipped, I rarely ever use them, I never really feel the need to to be honest :P But I mention it nonetheless.
Archwing launcher (PoE)
The Archwing launcher is quite handy in Plains of Eidolon since it summons an archwing where you stand (have to walk into the beacon to get it) and you can use your regular weapons for it AND do super cool 360 loops xD
But the main point of this is to travel across the plains in haste.
Fishing rod (PoE)
I’m guessing you already know what this does – if not – you can fish with it in Plains of Eidolon :P
Fish is required to be turned into resources required for the Archwing launcher ;)
Sometimes you will get blueprints to craft corpus units, infested healer units, grineer rolling balls, corpus drones, or other. These can after having been crafted, be summoned in a mission to aid you and your team in your efforts. Personally I think Drones and Infested healers are the most useful ones :) The drones give shields and the healers heal everyone :)