Mainly software for putting video onto the player, rather than from the player
I have been using Vuze [Formerly Azureus] which is a P2P torrent client but it includes a useful option to drag and drop video files into a TiVo section. It then converts the file so the TiVo can play it. These files can be accessed through the Now Playing page. The software also acts as a server.
The 2 downsides of this software for me:
It creates another, much larger copy of the file in a special transcode folder. Ok, you can delete this after you have transferred it to the TiVo but if you are doing a few, you can eat into HDD space.
Also the audio can be hit and miss. Some files have sounded a bit burbley
Also works well with the PS3.
There must be others out there, ideally one that would work with different media devices. eg PS3 and TiVo