There aren't many iPad apps that are all that helpful specifically for us freelance software developers. The main one I can think of offhand would be coincidentally the one that used the same name as my company, Codosaurus. It's an FTP client. There are a few SSH clients as well. But it's still not much of substitute for at least a laptop. However, I rarely have any use for those.
Other than them, I rely heavily on the Notes app just to take notes (been meaning to look into Evernote), and a standard mail client. The built-in mail app does OK, but G-Whiz is better for my purposes as it lets me use GMail's labels, which are critical to how I manage my email. (Long story short, I mainly use two labels, List and Work, automagically applied by filters, to divide my email into the four resulting combinations.)
Come to think of it, G-Whiz's interface to GCal is also useful since it pulls in other calendars a lot better than the standard calendar app, and I do pull in a separate "todos" calendar, my wife's, and a few more.