I own one for my Pi and it has become my main way of streaming Digitally Imported music around my house and back garden.
They come under different names i.e. Keene, B-Link or unbranded all the same internals but have near identical black casings with lettering and 2 metal strips.
One of the metal strips hides a loose wire this you can attach to a proper antenna to boost your signal strength.
*Keene have discontinued this product and have no plans to import more dongles.
I've seen some sold on Amazon or eBay however I would imagine you would have to bid for them now.
** Sadly the only eBay seller has increased his original asking price 4 fold $33 / £20, not only that but he does not ship to UK or Europe (USA is fine).
For what it's worth the FMBerry project with MMR-60 would be a much cheaper alternative, however this device does require an analogue input signal.
As it stands B-Link doesn't provide software support for Linux however there exists a 3rd party tool you can download and compile on the Pi - https://github.com/kenchy/keene-usb-audio
Unfortunately there are many features missing from this tool i.e. no mono / stereo switching, 75us pre-emphasis(treble boost) mixer volume control and tx power control etc.
So today I took the liberty of re-writing most of the code to enable all the missing features - http://pastebin.com/bZs8NYjW (copy and paste the text to a file called fmtx.c)
Currently the transmission range is set between 87.5MHz to 108MHz it is possible to edit the source code and extend this all the way down to 76MHz providing you're legally allowed to do so.
As for playing music you just need to select "B-Link" as your audio device (in qmmp you'll need ALSA Plugin set to sysdefault:CARD=Audio otherwise the music plays at the wrong speed).
For mocp (Music On Console) to use the B-Link audio device you need to create the file config in your .moc folder with the following line: AlsaDevice = "hw:1,0"