If a unit that can grapple is concealed and on the edge of an enemy's detection radius, using the grapple to move outside the range will not break concealment as long as the path of the grapple doesn't enter detection range.
Mimic beacons force enemies to attempt attacking it as long as they have LoS. This is very useful for flushing enemy melee units into the open for easy flanking shots.
Mimic beacons take full advantage of cover and can have abilities (such as Aid Protocol) cast on them.
Faceless ignore mimic beacons. (This only appears to happen to faceless that transform AFTER placing the beacon, thanks /u/Aedn)
You can cheese the avatar project by having blacksite missions readily available to start, but waiting until the project meter fills and the countdown begins before starting them. Essentially, only do blacksite missions after the countdown begins and as long as you can complete the mission, you'll never have to worry about the avatar project.
You can hold down "Control" and right click to set waypoints to create more precise paths for your units.
Placing proximity mines does not break concealment.
Enemy reinforcements do not appear until a full turn passes after an objective has been captured.
Reaching LoS (and therefor gaining control of) a "hidden" VIP will break a unit's concealment, regardless if there are any enemies in range. In addition, capturing any objective will also break concealment.
Unconscious units that do not make it to the Skyranger will show up as "captured" and can be reacquired as the VIP in rescue missions.
Knocking the floor out from under a turret is a guaranteed kill.
The insta-kill from a repeater can proc off the guaranteed damage from a stock and/or venom rounds.(A grenadier's saturation can apparently proc the repeater as well - thanks /u/C4ptainR3dbeard)
[BUG] If a unit next to a burning car gets picked-up by another and then moved out of the way, the explosion will still damage the unit that's being carried. Hopefully this will be fixed soon.
Enemies in the fog of war can still be damaged by AoE effects such as the EXO Suit's rocket launcher. And as long as you don't have LoS on these enemies after attacking, the pod will not "activate" until its turn begins.
You can uncover hidden faceless or chryssalids by using a battle scanner (or the similar specialist ability).
Enemies will still use AoE damaging attacks even if it means damaging one of their own.
If constricted, you can position grenades a certain way to damage only the viper and release the stuck unit.
If a unit is killed in an evac mission, you lose all their equipment unless you pick up and carry their corpse to the Skyranger.
Using threat assessment on an overwatched unit grants them 2 uses of overwatch.
Dropped items are picked up if they're still on the map when the mission ends. This is NOT the case for evac missions however.
The guaranteed damage from Combat Protocol removes overwatch.
(UPDATE
It seems my idea of "common" knowledge may not be the case for some, so I've decided to include some more well known tips in the list for easy reference.
Specialists gain 5 to their tech (hacking) score at the end of a mission if they successfully hacked something.[Thanks /u/rankzerox]
You can gain a bit more control on where a ranger performs their slash ability by moving the cursor to where you want them to attack from and then placing it over the enemy. If done right, the slash icon should appear along the movement path.
Making a double-move (dash) while concealed can "alert" the enemy to your relative location and will cause patrols to start moving toward the source of the sound. (Haven't had a chance to properly test this yet myself, but multiple users confirm it works. Thanks to /u/Niakshin for bring it to my attention.)
Calls for evac to the Skyranger last 4 turns. [Thanks /u/Binturung]
If you evac a unit currently controlling an enemy (either through mind control or haywire protocol) the controlled unit will break free. [Thanks /u/YourBlogSucksToo]
Dropping a gas grenade on a reinforcement flare will cause the units that spawn into to not move for an entire turn, allowing for some easy flanking shots! [Thanks /u/Snoozing_Daemon]
A faceless can detonate a vehicle if caught in the AoE range of its attack. [Thanks /u/Kouse]
Detonating a vehicle to kill an enemy without first damaging the enemy awards no XP or kill count. [Thanks /u/Grimy_Bunyip]
If a pod with andromedon stubles into your overwatch trap, and you kill the andromedon, the shell can attack you on the same turn it was revealed. [Thanks /u/Grimy_Bunyip]
Walking over a water/liquid tile while burning will remove the effect. [Thanks /u/Grimy_Bunyip]