There's a good list of 16 photography apps for the iPad on photography talk.com. I like many of these selections. Serious shooters should consider the upgrade to Filterstorm Pro that has better organization features and Raw processing, compared to the basic app included on the list.
Another app on the list, PhotoSync is really useful for moving pictures between devices, even in the age of Photo Stream.
One important omission, in my opinion, is Minimal Folio that is a terrific tool for organizing and displaying a portfolio of your best work.
The bottom line is, there are many good applications for photographers available these days. Peruse this list and see if something sings to you.
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Another important omission is Snapseed by Nik Software.
My "go to" apps are either Snapseed and/or FilterstormPro.
Will check out some of the other apps on the list.
Good point about Snapseed, Alec. I totally agree that it should be on the short list of photo apps.
They should add PixelSync ($10) for ipad to the list for Aperture users. It connects to Aperture via a desktp program, and lets you download previews for selected projects and (non-smart) albums to the ipad, along with the keyword, label, and rating, and caption metadata. You can work on the metadata, and then sync that back to Aperture.
There's a similar app for Lightroom, but not being a lightroom user, I can't remember it's name.