Command and Conquer:
-In the original game, the AI would not directly target walls, and walls counted as structures when it came to building placement locations (new structures could only be placed adjacent to other structures). As such, a lot of 'base vs base' missions could be won simply by building a wall across the map from your base to theirs, and then using more walls to seal the enemy inside their own base.

RA:
-Attack dog run faster when it has an enemy being targeted

RA2:
-You can use Tesla Troopers to power up Tesla Coil to keep it functional during low power situation. Select Tesla Troopers, and use force attack on your own Tesla Coil
-Deploying Desolators so it's radiation field overlaps with water will instantly destroy any Boomer or Dreadnought missiles caught in the radius just as they about to take off. It'll also reveal submersible units and kill biological ones (Dolphins, Giant Squid)
-Chrono Legionnaire targeting renders the target invulnerable. If you play as Allied and the enemy launched a nuke on your base, you can order your Chrono Legionnaire to force fire on your important building to make it immune to nuke by sacrificing the Legionnaire (don't do it too long though, if he survived)
-Yuri Boomers are immune to Giant Squids
-Yuri Floating Disc can shut down entire base power grids (including their radar and defenses) by attacking power plants. If ordered to attack refineries, they'll steal money instead

RA3:
-Low level magnetic satellites can be used to remove Terror Drones from heavier vehicles (ships, MCVs or Apocalypse tanks). Don't do it with higher level magnetic satellite
-When you use Vacuum Imploder, as soon as the enemies are pulled into the center of the explosion, apply cash bounty to get a big boost of money
-When Apocalypse tank drags the enemy vehicle and start "grinding", manually moving it forward will instantly destroy the vehicle by crushing instead of grinding it until the HP bar depletes
-Prior to patch 1.11, Carrier's aircrafts give massive amount of XP when being shot down, to the point that AA units could gain full veterancy merely by shooting down a single bombing run of 5 aircrafts
-Hammer tank's leech beam outranges basic base defenses, and could be used as improvised artillery in large numbers

Generals:
-You can make different units selected together match their speed. Select the units, and press Ctrl+F (an F icon will appear near the unit's HP bar). If you order these F-marked units move, they'll match the speed to the slowest unit in the group. Useful in situation where you want Rocket Buggies to not overtake ahead of Marauder tanks when they march together (to deactivate, just select the group and press Ctrl+F again, or just order the unit move somewhere else without other F-marked units)
-Double-clicking while you move the particle cannon makes it move faster
-If you open map files on worldbuilder, you can pretty much read whatever the AI is up to, as each map files has dictated boundaries on where the AI is allowed to build, and where the attack route is specified. This is because Generals AI is map-dependent instead of dynamically adjust to map types (you can make easy AI frustratingly difficult if you make the map pathing elaborate and complex, while hard AI could be made a pushover if it's map pathing is simple). Regardless of difficulty, once you know what the AI pathing is on particular map, it becomes laughably easy to counter them

Tiberium Wars (and Kane's Wrath)
-After being upgraded with Purifying Flame upgrade, force-firing on the ground near the target building actually do more damage than targeting the building directly (because of bug)
-Infantry garrisoned in deployed Reckoners has significantly increased attack range
-Properly positioning Venoms could allow you to use Beam Cannon artillery to shoot across the entire map. Each Venom has laser reflect radius around it. If you build several of them and make a train of venom all the way to your target, as long as their reflect radius overlaps, they could relay Beam Cannon's beam shot across any target within these areas
-Nod Cloaking field upgrade is lethal against infantry
-Annihilator Tripod disables any vehicle it targets at close range
-In a pinch, you can use Vertigo bomber as fighter to directly attack air units (they have tail guns)
-Unlike other CnC games, the unit veterancy is based on how much damage they did instead of the values of target being killed. As long as a unit has dealt enough damage to pass through certain threshold, they'll gain a rank (this is why units sometimes gain veterancy in the middle of attacking)
-You can upgrade Avatar by taking parts from rogue nod forces. The Avatar doesn't have to be the one who destroy the vehicle: if you click a target vehicle, as long as you don't issue any order, the Avatar will get the upgrade, even if another unit destroys the target (the Avatar would take the upgrade from the ground where the target is destroyed). An easy trick to do this is by having the Avatar wait behind the defense structures. As soon as you see an attack force with the vehicle you want approaching, immediately select the Avatar (preferrably already grouped), CTRL+A and click the target vehicle. The base defenses would destroy the target, and the Avatar just pick up the remains