Kinsky by Linn can do this across all platforms. It is in effect a uPnP control point.
For Android there is BubbleUPnP. This is capable of serving all DLNA/UPnP purposes outlined below, and is an excellent tool for carrying out tests.
On linux there is the gupnp-av-cp app that is part of gupnp-tools. I have read that it's also possible to to use pulseaudio (with rygel?) to push all system sound to a UPnP/DLNA device and am in the process investigating/implementing this. Will update when I know more.
On OSX 10.8, iTunes is capable of sending to both AirPlay and UPnP/DLNA devices.
To put this in perspective, DLNA, also referred to as uPnP, has three important devices:
- Media servers
- Media renderer
- Media control point