Tvheadend provides a HTTP streaming mechanism that can be used in conjunction with many different media players. However several clients also include support for the HTSP protocol that Tvheadend uses for both streaming and control, these give a much richer experience.
Below you will find a list of a few that we are aware of, there may be more!
Showtime was the original Tvheadend client, written by the original Tvheadend author, Andreas. It is still one of the best clients for interacting with Tvheadend.
With the integration of PVR functionality into XBMC, this has now become possibly the most popular Tvheadend client. Indeed much of the recent development of HTSP has been focused on improving integration with XBMC.
An Android app, written by John Tornblom, that can be used to control Tvheadend and also stream TV services to your phone.
This is a dedicated Tvheadend client, written by Dave Chapman, specifically for the raspberry pi. It aims to create a very simple, intuitive user interface similar to that you might expect of any off the shelf STB.
This VLC plugin integrates HTSP support, thus allowing a richer interface within VLC and thus turning it into a stand-alone Tvheadend client.
An iPhone app, written by Luís Fernandes, that can be used to control Tvheadend.