It really bugged me that there was no iTunes for Linux, and despite searching high and low, there is no decent alternative that works on Linux - a simple way of buying a single track online. Until now.
I've just found and installed PyMusique. Thankfully, it has Ubuntu packages, which work perfectly. The app is very simple, but very effective. It allows you to sign up, add credit card details etc, search, preview and purchase tracks all very easily. I believe this is due to the iTunes DRM being cracked earlier this year, but I didn't realise it was now available in such an easy package. I'm now listening to "I'll stand by you" by The Pretenders, one of my all time favourite tracks which annoyingly wasn't in my music collection, and I wasn't going to fork out for an entire album, or download it illegally using the P2P networks. Kudos and thanks to the PyMusique team, and particularly Jon Lech Johansen, to whom I also owe the ability to play DVDs on my Linux PC.
Update: I found a script to convert the iTunes m4a (mp4) format in Ogg (with the help of faad and oggenc), and now I can play them in my favorite music player Amarok (which doesn't seem to be able to play them with decent quality. I did have some issues with the debs from PyMusique - in particular I ended up installing gstreamer0.8-faad and libfaad2-0 from marillat's Debian repositories, instead of using the PyMusique ones. But now it's perfect - I've got a nice frontend for buying from iTunes, and Amarok is a great player for organising and playing music.