One of my transaction was stuck month ago that time I came to know about these services from quora post then google search. I used all of them (Free only) and next time when I checked transaction was confirmed.
Aprtfrom this you can also use Binance's btc tx accelerator its free for VIP members but they charge free to other I guess.
The only free bitcoin accelerator is
https://www.viabtc.com/tools/txaccelerator/. Other bitcoin accelerators are either not free or they are scam.
If your transaction is not having more than 500 bytes in size and the transaction fee paid is not less than 10 sat/byte, you can use ViaBTC free accelerator.
Others that call themselves free accelerators will keep rebroadcasting your transaction and not accelerating anything.
Is there any way to check if it actually works? I've used ViaBTC accelerator for a bunch of times and it always took hours to get a confirmation even after transaction was already accelerated. So in my case it was kinda impossible to check whether it could take longer to get it confirmed.
It is much better just to change the transaction fee via replace-by-fee instead of relying on accelerators.