I'm not a big fan of "free actions" in GURPS.
Don't get me wrong - some things should take so little time that they're effectively an extra you can throw in.
But I've found the term gets tossed around a little too freely in my games sometimes, and it's used to allow a lot of actions.
If it's Free, it's for Me.
Here are the canonical free actions, all on Basic Set p. 363 unless noted otherwise.
Talking. Speech is free, which is an appropriate idea for a game written in the U.S. of A. But you have one second . . . the rules account for this but say it's more fun to ignore it. I can vouch for the fact that this is not always true.
Maintain Spells. Not cast or change them, but you can pay the FP cost if a spell needs it.
Dropping things. You can drop something as a free action, at any point in your turn. Related - you can let go of a hold as a free action, too (or throw a weapon away from you, nevermind just drop it, per Actions After a Grapple, per p. B371.) Letting go of picks is a free action (p. 405).
Crouching. You can crouch at the beginning of your turn, or rise from a crouch. The "beginning" of your turn isn't defined. What counts as enough action to end the beginning? Presumably, not any free action, but any other action at all even if it's instant (see Problems, below.)
Drag someone. This is described as a "free action" on p. B371, but moving is not a free action, so clearly this is meant to just be an extra you get with any Step or movement if you qualify for it.
Speaking of grappling:
Adding a hand or Releasing a hand are both free actions, per Martial Arts, p. 117.
Activate a laser sight. Per p. B412.
Tonfa grip change. Per p. B209. Oddly, this is worded differently from Fast-Draw - it's free, or a full Ready.
Dissipating a held Melee or Missile spell. Per p. B241.
Dismissing a summoned Ally. Per p. B38.
Raising or lowering a Mind Shield. Per. p. B70.
Using Rapier Wit. Per p. B79.
Hit someone with spines. Per p. B88.
Adjust your grip with Grip Mastery. Per Martial Arts, p. 50. Interestingly, this is one of the few free actions that comes with a limit on use, like crouch does.
Quick Sheathe with Fast-Draw. Like Tonfa, this one on Martial Arts, p. 51 elevates a Fast-Draw to a free action on a success.
Quick-Swap weapons. Per Martial Arts, p. 51.
Use a Schtick! Per Martial Arts, p. 51
Brace a teammate. Per Martial Arts, p. 52, if you both have the Teamwork perk and are formed up.
Pin after a Piledriver. Per Martial Arts, p. 86.
Switch skills. Per Martial Arts, p. 104.
Drop into kneeling or prone while skidding. Per Martial Arts, p. 105.
Use Light Walk. Per Martial Arts, p. 129.
You can do any and all of that, given the right circumstances.
There are two problems related to this:
The everything fast is free. I've very often heard players refer to instant actions as free actions. In other words, Fast-Draw? It's a free action. "And I Fast-Draw my sword as a free action." Well, no, you don't. You attempt to ready it with a roll; if you succeed, you ready it instantly. But it's not technically a "free action." This matters, because of rules like Multiple Fast-Draw (Martial Arts, p. 103), and because failure means you're forced to take a Ready next turn.* This is important - so if you're doing another action, like a Long Action digging for something in a pouch, you can't Fast-Draw as a "free action," unless you've got a hand free and it has consequences for failure.
Also, it leads to the assumption that any other sufficiently fast action doesn't count; what's "sufficiently fast" seem to vary by player. "I can take a Wild Swing** as a free action, right?" No. You can do a Move and Attack, and any attack that's sufficiently difficult might be a Wild Swing, but that fact that you can attack on the run (or just run) doesn't mean it's a "free action."
You also get the idea that any action not specified to be a maneuver is actually free. Perception rolls? Free, how can that be an action? Facing changes come with Movement across a hex map, and Step comes with facing changes, so they must also be free. And so on. It shouldn't take long, so it's free.
Layered "free" actions."
In other words, free+free+free=free.
This means you get the guy who does all of this:
"Okay, I've going to step over here. I drop my sword to my lanyard, Fast-Draw a potion, drink it***, drop the vial, yell over to [so-and-so], 'We need to gang up on this monster, so flank him and use cutting because I think he's not really hurt by impaling,' Fast-Draw my sword from my lanyard, and yell to the high priest, 'I'm going to kill you!' - hopefully that will distract him, do he get a roll to avoid that? No? Okay, fine. Oh yeah, I drop my buckler and Fast-Draw my other sword with my off-hand. Then I turn my facing over to here so I can't be flanked by that guy and so I can see my buddy [other so-and-so]. Can I make a Perception roll to see that he's been poisoned? I mean, it should be obvious. Oh, and then I crouch."
Nevermind you can't do the last one, but I've seen "crouch" as a free action become "crouch or un-crouch at any time."
I'm barely kidding here. We've had people concentrating on multi-second spells wanting Perception rolls to spot things and make multi-sentence speeches. Technically the second part is free, but it's not actually reasonable. (And it gets worse if you add Great Haste to give a full additional turn.)
It's one second . . . and it takes more than one second to even think of doing all of the above.
If this kind of thing doesn't bother you, go with it as above.
If it does . . .
Limited speech. You have one short sentence. Have a speech? Make part of it, make the rest on your next turn.
Limited free actions. Just cap the number at 1-2.
Cumulative free actions count. Allow one of the free actions per turn without a penalty plus one-sentence talking; all others get a cumulative -4 to rolls a la Multiple Fast-Draw. So if you uncrouch, talk, drop a weapon, and then go an Fast-Draw a new one and attack, you're at -0 for the crouch, -0 for speech, -4 for dropping a weapon . . . so your Fast-Draw is at -4. You then attack at -4. That, of course, makes "free" not free, and not-free better. You can just make it a -2 for each one, instead, and count them all. Crouch, talk, drop = -4 to Fast-Draw, -6 to attack (all attacks given by Extra Attack would use this same penalty.)
Too much counting? Any free actions past one means a total -4 to everything else on that turn. This makes a single action a nice bonus, but then you're just distracting yourself trying to do too much.
If you think Free Actions get too much abuse or even if not, it's worth reviewing what is actually free.
If you think they're abused, try the limits I mentioned above. You can always be flexible if the circumstances dictate, but it's harder to suddenly be less flexible.
For me, I found it interesting to just track down the words "free action" in the core books (Basic Set, Martial Arts - Magic has nothing Basic Set does not.)
I'm not sure how I'll rule on these, but I do like the idea of limits. One second is just so much time . . . and it's getting tiresome to deal with talk-step-drop-fast-ready-attack-drop-talk-turn-look over and over.
Hopefully this look at free actions is helpful to people besides me.
* Which is occasionally an issue, when people read this as "Success means it's out now so I can parry; failure means it's not out until my next turn, which I can use normally." Failure is so infrequent at higher power levels, such as my current campaign, that it doesn't come up much. But I have retroactively realized someone has failed Fast-Draw and then acted normally on the next turn, having just not parried in between.
** Usually they mean a Move and Attack.
*** My house rules allow a one-second open-and-quaff for potions. I recommend giving it a try.