Most of the "gadgets" that would do this aren't cheap. At least not the ones I'm thinking of off the top of my head. Example, I have a Playstation 3 that will play my shared Windows Media Player 11 library from my desktop PC across the room. Xbox 360 will do the same thing (I think).
For just music, if you use iTunes, you could get an Airport Express to stream audio (only) to a stereo over the wireless network.
I don't know about playing a DVD on a laptop wirelessly to another device though. I can't think of any way to do that one.