Useful Combat Macros
Macros have uses during combat but they should be used cautiously or it will be detrimental to your output. Any actions on the global cooldown (GCD), i.e. weaponskills and spells, should not be assigned to a macro, because it will cause a delay when executing the macro[1] and ultimately result in lost casts over the course of an encounter. However, it may be recommended to macro some off-global cooldown (oGCD) actions, particularly those that require you to target another party member, to avoid the hassle of having to manually switch targets.
Targeting other players
The general structure for a macro that targets another player with an ability is:
/ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /ac "action_name" <#> /micon "action_name"
Where action_name is the name of the action and # is the target. # can be set to a number (2, 3, 4, 5, 6, 7, or 8) to always target a person based on your party list, or mo to target a player you hover your cursor over, either their actual character or their spot on the party list.
While the number targeting can be more consistent depending on your preference, a downside is that a separate macro needs to be made for each action for each party list number (2-8) which will take up a lot of limited hotbar and macro storage space, along with possibly cluttering your user interface.
The /ac line is repeated multiple times to ensure the action is "queued" correctly.
Ground targeting
Some actions require ground targeting, which can also be finicky. These actions include 
Asylum (WHM), 
Liturgy of the Bell (WHM), 
Sacred Soil (SCH), 
Earthly Star (AST), and magical ranged DPS 
Limit Breaks.
Instant action
/micon action_name /ac action_name gtoff
By taking advantage of gtoff, we can target instantly at the cursor location without having to go through ground targeting.
Targeted action
/micon action_name /ac action_name <t>
Since in many encounters, fighting occurs around a specific target, like the main boss or the tank pulling a pack, a targeted action can also be used to automatically place it at your current target. Be aware that there are some situations where this is not appropriate, such as if the boss is located outside of the arena or if the boss is about to move.
This is most useful for controller players, as it may not be as precise to use the gtoff macro above with the controller targeting mode.
Tanks
Shirk
During 8-man content requiring a tank swap, macroing 
Shirk will circumvent having to target your co-tank. The following macro assumes that your co-tank is number 2 in the party list. If not, replace it with the appropriate number (3-8).
/ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /ac "Shirk" <2> /micon "Shirk"
Holmgang (Warrior)
Holmgang requires a selected target to cast, unlike other Tank invulnerability cooldowns. If the target enemy dies, the player will lose the effect. Also, if the target is out of range, the player cannot use this ability. To circumvent these quirks, it is recommended to use a self-target macro, e.g.
/ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /ac "Holmgang" <me> /micon "Holmgang"
Targeted actions
Targeted mitigation and healing abilities, such as 
Cover (PLD), 
Intervention (PLD), 
Nascent Flash (WAR), 
The Blackest Night (DRK), 
Oblation (DRK), 
Aurora (GNB), 
Heart of Stone (GNB), and 
Heart of Corundum (GNB) can be assigned via the general "Targeting other players" macro format above.
Healers
Player and ground targeted actions
Healers have a variety of player- and ground-targeted oGCD abilities that you may decide to macro if you prefer. See the "targeting other players" and "ground targeting" sections above.
Resurrect
Resurrect actions, such as 
Raise (WHM), 
Resurrection (SCH/SMN), 
Ascend (AST), 
Egeiro (SGE), and 
Verraise (RDM) should not be assigned to a macro because they are global cooldown actions. However, some players decide to attach a text macro when they revive another player for the purposes of letting other jobs with resurrect actions know that the target player has been revived to avoid overlaps. This text macro should be standalone, e.g.,
/p Reviving <t>
In general, make the text concise and informative, and avoid attaching sound effects (<se.#>) or passive-aggressive messages which can annoy other players.
DPS
Actions such as 
Dragon Sight (DRG), 
The Warden's Paean (BRD), 
Closed Position (DNC), 
Aetherial Manipulation (BLM), and 
Rekindle (SMN) may benefit from the player targeting or mouseover macros described previously.
- While only once Dance Partner is usually chosen for a given encounter, there are situations where it is beneficial to switch partners, such as if your current partner dies.
Shukuchi relies on ground targeting and may also benefit from a ground targeting macro described previously. It is recommended to keep a non-macro version of Shukuchi assigned for situations where you need to use manual targeting.
Magical ranged DPS Limit Breaks may also benefit from the ground-targeting macros described previously. In situations where precisely hitting all targets is critical (e.g. the Cosmo Meteor mechanic in 
The Omega Protocol (Ultimate), using a targeting macro is recommended over manual ground targeting.
Fight-specific text macros
Some fights, especially those with memory-based mechanics, may benefit from macros that remind players of mechanics. For example, if the boss uses an ability that stores either a spread or stack mechanic for later, you could make two macros: one for spread, and the other for stack, then press the appropriate macro. That way, no one needs to remember this initial stock cast.