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.
This is right and I just want to add that there's another requirement too.
If you want to use ViaBTC free accelerator, your transaction must not have any unconfirmed parents.
This (confirmtx.net) or this (confirmtx.com). Both sites don't work... lol
I just visited the former and it was more than clear that it was a scam.
I entered a random transaction ID in their website and it gave me different options. It asks $999 for getting 3 confirmations in 60 seconds, $500 for getting 1 confirmation in 5 minutes, $50 for getting confirmation in 11 minutes, $25 for getting confirmation in 30 minutes and $15 for getting confirmation in 1 hour.
To any newbies reading this post:
There is no way to give a precise time for transaction confirmation and the website I wrote about is a scam.
As already mentioned by Charles-Tim above, ViaBTC service is the only working free accelerator.