Every since my first MP3 player, I've longed for the day when my player would have capacity enough for all my music and that I could load new songs automatically without having to hook up my device. As players have become larger in capacity, the first issue isn't as much of a deal anymore, but the latter issue is still a concern.
About two weeks ago I found a free app called Zumocast. This has been mentioned in other posts on this site, but mostly in reference to streaming videos. It's great for that too, but since that's been covered, I won't spend any more time talking about that. I will elaborate about the music steaming however.
With a free download on my computer (a program which essentially creates a server on my system) I can steam all my music and playlists over wifi AND 3G! Let that sink in. Via my iPad (and it works for the iPhone too) I can listen to any music I own just about anywhere, and there's no loading the music onto my device.
After having tried this with the iPad, I excitedly bought an 8GB iPod. For the first time I have an MP3 player where the capacity does not matter, as almost all my content (save apps) is steamed from my computer. Should I move away from a wifi area, I can download music and videos onto my device wirelessly (assuming I do this while in my wifi area - again, this is not an issue for 3G iPads) and listen to the music on my player. The Zumocast website forums seem to indicate that you can't do this on an iPod for some reason, but I know that's wrong because I just did it on my player.
I can only see two concerns here. First, if you like using the iPod player you are out of luck. You need to use the Zumocast player, regardless if the media is streaming or downloaded. I only use the Zumocast player, so this isn't an issue for me. Second, the app and service is free, including no ads. That may not sound like a bad thing, but it raises two issues for me. One, it makes me wonder if there will be a charge later, and will I want to pay that charge, and two, if there isn't a charge, will this service eventually go away because the company isn't making money to maintain it.
For now though, this is something I've been waiting for for about a decade, and I highly encourage you to check it out. (incidentally, I have no association with this company whatsoever)
Oh, and by the way, you can wirelessly sink non-media files from you computer as well.