What applications of clients use bitcoin JSON-RPC? As far as I understood, the Bitcoin JSON-RPC is one of the ways (if not the only way) in which applications can be built that can access the bitcoin wallet. So, the RPC server here gives that access and any client (or app) can access via APIs.

So far, I have just figured our that only bitcoin-cli is such a local application which uses the RPC. Are there any other examples where the JSON-RPC is used? 

In other words, are there any Desktop apps (mainly) that can access the bitcoin wallet.
As far as I understood, the Bitcoin JSON-RPC is one of the ways (if
  not the only way) in which applications can be built that can access
  the bitcoin wallet.


There is also a REST interface, but that is for read-only type commands like getting blocks or transaction info.


  So far, I have just figured our that only bitcoin-cli is such a local
  application which uses the RPC.


bitcoin-cli is just a cli interface to the RPC, it doesn't really do anything on it's own. Also, the RPC interface isn't just for wallet commands. A list of commands can be found in the documentation at https://bitcoincore.org/en/doc/0.16.2/. The commands expose functionality including:


blockchain
control
generating
mining
network
rawtransactions
util
wallet



  Are there any other examples where the JSON-RPC is used?


Many software libraries will communicate with the Bitcoin Core RPC, too many to list, but this might be a good start: https://github.com/igorbarinov/awesome-bitcoin. Not all use the Bitcoin core RPC, some implement their own full node (like copay).