Give them to those who need them.
First of all, novice students in your gym or mountaineering school will be happy to use a bit heavier/older gear, but ease the burden of buying everything for their first trip.
Just make sure that carabiners are safe, because the newbies can't tell themselves.
Then, any local non-commercial groups, who organize rope fun for kids or everybody.
A hand-made fun park needs a lot of stuff.
Finally, if you are heading to a remote Asian mountain region this summer, you can take carabiners and old ropes, which the locals will happily use for many years in their households to tie their livestock, etc...
Of course, this is tricky because of limited airplane weight.
outdoors.stackexchange.com/users/1794/Steed
You probably already do this, but make sure you mark all 'retired' carabiners with their own color of tape so that you never accidentally climb with one.
Then let the fun begin.
If you do any backpacking, they are extremely useful for attaching things to your backpack.
A few other favorites: dog leashes, making clotheslines, or use with hammocks.
If you're feeling adventurous, maybe chain a few together and assemble lights to hang from the ceiling with said chain.
outdoors.stackexchange.com/users/1975/Scott Hillson
As Hillsons suggested a good way to use it, let me put in what I would do: You can use it as a Weight.
Simply knot a carabiner to the end of a rope that needs slinging over a pole or onto a roof.
The weight helps with accuracy.
Clip keys inside backpacks to loops for extra security Link a few together for your kid to play with while you are shopping.
Attach your water bottle to your backpack.
Hold electrical cords in a loop (Don't like to have cable-tie?)
Tie the neck-belt using the Carabiner to secure your pet to an anchor.
Tie up all your key-chains to it and hang it using a sling?
To hang whatever stuff
you
wanna hang.
Please bear in mind load specifications.
outdoors.stackexchange.com/users/2303/WedaPashi
Obsolete biners are very useful for retreating.
If (as you state)
you've retired them because: made obsolete by an upgrade.
and they are generally solid.
Then it's well worth dragging a few of these along on climbs
you're worried you may not be able to complete.
I've been welcome of some old steel biners when retreating off of climbs in the past .
They're solid and dependable and I don't mind loosing them.
Well worth the extra weight.
outdoors.stackexchange.com/users/None/
You can drill some holes in your carabiner and use it as handle for mugs and other things in your home you need to lift. mug example
outdoors.stackexchange.com/users/3605/ibex
This is probably going to be an unpopular opinion because it's not nearly as "feel good" as helping remote Asian herdsmen.
(Which incidentally is probably one of few exceptions to what follows.)
Nevertheless:
It is my belief that unsafe carabiners should be destroyed.
In my opinion it is too great a risk that someone will get ahold of one and see that it has the right marking to be a "real" carabiner and use it for some kind life-support application.
Mind that I am strongly opposed to excessive safety regulations and
I firmly believe in using your brain and "swim at your own risk" etc.
This
notwithstanding
I feel that the risk outweighs the benefit when it comes to distributing unsafe carabiners.
(I also realize that even when new carabiners are dangerous for those who do not know how to use and inspect them but people do not usually give away brand new carabiners.)
For the uninformed there may be certain expectations of climbing equipment.
Just as "a little knowledge is a dangerous thing" so too I believe is a bit of old climbing equipment in the wrong hands.
Worse, the "real" equipment may give a sense of validation to ignorant use that results in greater risk-taking than if hardware-store equivalents were used.
Now if the old carabiners are truly still safe to use then use them!
If your equipment doesn't wear out before it truly becomes obsolete you should make it a mission get out and climb more. :-)
outdoors.stackexchange.com/users/133/Mr.Wizard
If you're handy with welding (or a drill and screws, brackets, or a hot glue gun), you can make a pretty cool coat rack or key rack out of old carabiners.
You can remove the gate for a coat rack by knocking or drilling or cutting off the pins, or leave them on if you're making a key holder.
Just attached the carabiners with the hook down and out from the wall by welding, gluing, bracketing, or drilling and screwing it onto some form of a back plate.
To address the safety issue: once carabiners have been mechanically attached to a wood or metal plate and hung on a wall, no one will identify them as serviceable climbing equipment (except for children, who would probably recognize dental floss as suitable rope for rappelling off the roof before they could remove the carabiner from your wall).
outdoors.stackexchange.com/users/3626/pheidlauf
A nalgene water bottle holder!
You can then clip it to the back of your pack, and it helps with carrying it around!
You may want to decorate yours though.
Also caribiners are nice just to always be attached to your backpack for anything with a loop!
outdoors.stackexchange.com/users/3375/Liam McInroy
Get a set of curtains and make the carabiners the rungs for the curtain, just make sure that the bar that it slides on is of a good diameter for the carabiners, or if you can find a bar sturdy and narrow enough you could take the core out of a retired climbing rope and put it over the curtain bar for bonus points.
outdoors.stackexchange.com/users/14591/andrew
"Make a Dog Lead" has already been said, but I use an ancient carabiner at the other end of the lead to clip it to my belt.
This enables hands-free dog walking , which permits me to put my hands in my pockets while out with the dog, Especially good in sub-zero winter conditions.
If you haven't got enough dogs, then your local dog pound might appreciate the donation of clips and rope that is no longer suitable for climbing.
outdoors.stackexchange.com/users/11223/Criggie
I seem to have accumulated a good number of carabiners that, while probably still safe, I won't climb on anymore. They might be worn, of questionable origin, or just plain made obsolete by an upgrade.  What are some creative uses for these carabiners? I already have plenty of shoe and leaver 'biners, so non-climbing uses would be most useful.
What are creative uses for retired carabiners?
outdoors.stackexchange.com
<gear><climbing><carabiners>
outdoors.stackexchange.com/questions/6034
outdoors.stackexchange.com/users/671/Felix
Carabiners can be used to make coat racks, handles for mugs, curtain rungs, attaching things to a backpack, tidying electrical cord, attaching keychains, making dog leashes, hammocks and clotheslines. Unsafe carabiners should be destroyed or marked as retired. They can be repurposed in several ways, including for attaching water bottles to backpacks and for dog leads.
Carabiners can be reused safely for numerous purposes. They can be used for attaching water bottles to a backpack. Unsafe carabiners should be destroyed or marked as retired. You can make a dog lead with them.