No matter what kind of beats you're making, I strongly recommend DCAM.
Video tutorial and online review for iSequence, one of the best the apps that let you make songs right on your iPad.
Check out my DubSpot review of Propellerheads' software that changed the world: ReBirth-338 for iPhone.
iPad app reviews: TouchOSC is control for your computer. Sonorasaurus a horrible DJ app which shows us that although the iPad is great, it is not ready for native DJing yet.
iElectribe and Looptastic HD are two of my favorite music apps for the iPad because of their sound quality, ease-of-use, and features.
Novation has managed to make the transition from synths to controllers in style by creating Automap.
One of the coolest controllers to come out in the past few years is the APC 40, a device made by Akai and Ableton.
The Roland V-Synth GT is a sound designer's dream, with an easy to use interface and a powerful synth engine under the hood.
Access has been making awesome keyboards for years. This is the Virus TI, one of their new generation models. It combines the classic Virus sound and hardware with softsynth ease of use.
Ableton Live is one of the most unique and widely used sequencers on the market today.