The two different Add-ons to Terran Factories: one allows you to build 2 units at the same time (useless since it only allows 2 Hellions at a time). The other allows you to build Siege Tanks and THORs.
Marines with shields. Shields add 10 HP to Marines.
The different Barracks Add-ons and a Missile Turret at the bottom left:
A Sensor Tower (selected)
No one uses this thing, it costs 100 minerals and 100 gas. What it does (hard to see in the screen shot) is show you enemy units as pings on your screen even if they're still in the fog of war. Sounds pretty good, and worth the 100 minerals/gas, right? It also makes a big circle on the mini-map of all the players in the game, the tower being the center of the circle.
Starports and their Add-ons (one allowing 2 units to be produced at a time, the other giving access to Banshees, Ravens and Battlecruisers (with Fusion Core):
This Command Center has been upgraded with Planetary Fortress, and has a THOR guarding it:
Siege Tanks (one in siege mode, one not) around a bunker at a ramp):
That bunker can be upgraded to hold 6 Marines instead of 4:
Command Centers can also load SCV's:
You can do this to save them from being attacked. You can also build a Command Center, load the SCVs, move that CC somewhere else, and unload the SCV's to have instant workers.