Akai Professional Synthstation 25 by xblindxx
I recently got this keyboard for the purpose of sketching ideas on the go with my iPhone. One thing I liked about it compared to other 25 key USB keyboards was the pitch and modwheel being on it, which are actually pretty solid when used.
In use, the iPhone apps are actually hard to use on the small iPhone screen (stuff like music studio which is great, but better on the iPad) but this controller definitely helps since you can just perform parts in to a metronome.
It is advertised to use the synthstation app but from the reviews it is not really worth your time. It is a few extra bucks in the app store but I don't see it's real purpose but to add more synths to your arsenal. I haven't actually tried it so I can't say so myself, but would imagine the reviews are correct.
One of the biggest reasons I got this was to have a usb midi keyboard that could fit in my backpack with my laptop so I could use it as a basic midi controller with cubase and ableton which it works great for.
It is battery powered (4 AAA) but it also has a dc power jack which you would have to buy separate. The biggest disappointment, however, is that it isn't powered by USB. I can see why they wouldn't have it as the focus since the iPhone/iPod touch was the selling feature. It does have a headphone out which I use, but it can feature unbalanced RCA outs for speakers if you desire.
Overall, for being under 100 quid, this keyboard is really a good buy for on the go situations, even if it isn't for the apple products. Recommended.