I run a headless version of Transmission on my router -- RxTx aims to implement basic functionality needed to operate the Transmission daemon remotely with its RPC mechanism.
Tx sends a request to start download of a torrent. When downloading .torrent files, I use Tx to open them and then they're downloaded on the remote machine.
The idea is to implement some simple callback functionality in (headless) Transmission that sends an HTTP request when a download has finished. Rx will be a simple server to receive notification when a download is completed. One use case is to forward the message to Growl.