iPad apps I use for DJ related stuff:
MOG - music subscription service similar to Rhapsody, Spotify, Napster etc. I like this one because I can download song files in full 320 kbps quality to iPhone or iPad, and play them from the disk. Even over 3G at events I can get any song in full quality pretty quickly. At this time there is actually only an iPhone app but I like to use it on the iPad to free up my phone for other stuff. The bigger screen makes it easier to search and play music.
Form Tools: For contracts. Import your blank contract as a PDF. Create input fields where the text goes. When you sign a contract, quickly type in the information and they can sign it right on the screen (a stylus is great). You email them a copy of it on the spot.
Invoice2go: Same as contracts, paperless and on the go. Handles all accounts and invoices pretty well as far as I'm concerned. It even adds a Paypal button so they can click on it and pay you. Again, take the deposit and email them an invoice right on the spot. I like to meet couples at Starbucks.
Dropbox: Sync documents between your computer, iPad, and iPhone. Stored in the cloud. 2 Gigs free. A great way to get PDF documents, pictures, song files, anything you want on your iPad or off. For example, in the earlier example how do you get the contract onto your iPad? Email it to yourself? Sync with iTunes? Not necessary. Dropbox is awesome and free.
LogMeIn: Great remote access tool, control your computer from anywhere.
Other iPad apps I like: iCabMobile Browser, Alien Blue HD (Reddit Client); DocsToGo, BillMinder, Intellicast HD, Instapaper, AppAdvice, Flipboard, eBay, NPR, DocScan HD, Penultimate, Skitch, mSecure, Printer Pro.