Whatever project you're making, the Arduino is one of the best platforms to realise your idea. With it's ease of use, the ability to communicate with Ableton Live via Max for Live, the plethora of components and the fact you can hack it to be a MIDI controller, there's no stopping the hacker-musician from creating their dream device.
That's exactly what Ryan did, and now he has his own, custom step sequencer: the RN-9090. The RN-9090 was created to be a controller for his 909 clone - Project 9090.
Although looking similar to the Arduinome, the two devices are very different. The RN-9090 is a pure MIDI sequencer. Unlike the Arduinome, it doesn't interface with Ableton natively, just other MIDI devices. It uses it's own internal clock, which stops it relying on other devices.
A novel feature of this step sequencer is that it uses a "pages" feature, so that you can view and edit one pattern, as another is still playing. You can also link the 2 pages to create a 32-step sequence. The 128x64 LCD screen shows vital info, such as tempo, current playing/editing page and save/load options.
The custom case looks ridiculously professional, down to the classic Roland fonts. It looks like a commercially available controller, not something made with an Arduino and some Sparkfun buttons!
Ryan is in the process of creating an updated version with detailed build instructions and a parts list so you can create your own. A bit wary of making your own? He will also be creating a limited amount of pre-built sequencers. If you want to get hold of one, his contact details are on the RN-9090 detail page.