IPad – n iPad app like Snagit for marking up screenshots or pictures with arrows, callouts, dropshadows, torn edges, etc.

With a little hunting around, watching videos, and emailing developers, I've come up with the following list.

• iAnnotate. This has good annotation (obviously) and when connected to an external display then the display version has no page transitions (note that there are transitions on the iPad display, though they rate very low on my "irritatometer"). Other pluses: customisable toolbars, and the presentation display doesn't show the toolbars or menus. There is a slight oddity: even though the presentation display doesn't show the toolbars, the size of the document depends a bit on what toolbars are on display on the iPad.

• 2screens. The focus of this is on the presentation, which it does extremely well. The annotation works in a slightly different way to other apps. Imagine a glass sheet in front of the presentation and that you are writing on that, not on the presentation itself. Once you get used to that (so saving and blanking the annotations when shifting slides, I've been told that the developers are working on making this automatic) then it works well. The presentation part is very nice, with none of the menus displayed on the external dispay.

• Explain Everything. This is, perhaps, a slightly surprising contender as it is really for recording a short presentation (or other). But it can also be used for the presentation itself and has annotation support. The external display mirrors the iPad display, but there is a "minimal" toolbar which is very discrete and contains all the main presentation controls. The only snag with this one is that there is a strange offset with the PDFs that I use (created by xelatex) which means that the slides get successively more out of sync with the backgrounds. The developers tell me that this depends on the PDF so other methods of PDF-generation may not have this problem (and I'm hopeful that it would be possible to post-process the PDF from xelatex so that it works as well).

• GoodNotes (formerly eNote Taker). This has a little history! When it was eNote Taker then it worked just fine. In the upgrade to GoodNotes, sliding page transitions were introduced. In the current version, tapping on the screen changes pages without the sliding transition so it is usable again. The display mirrors the iPad display, but the annotation toolbars are very discreet in that they collapse out of the way when not being used.

No. In short.

The longer answer: All apps in iOS are sandboxed, meaning they effectively have no permission to do anything other than that which they were written for, other than through specific exceptions through API calls to other parts of the OS etc as allowed. For example my Twitter app is allowed to access my contacts to use and set contact details as is deems fit. Many apps (in my case Instagram,Tweetbot and Youtube) are allowed to send tweets through the Twitter App, but in all these cases (or any case where one app access some data or functionality provided by a second app) this is not a system wide availability. Using said APIs an app can make portions of itself available to other apps, but unlike Quicksilver/Alfred/Launchbar on the Mac, this must explicitely be defined by the target app. So taking for example the "Due" app for iOS which is a reminders app, it has an API that will allow other apps to create reminders in it, but that is all that other apps are allowed to do, and in order to do so they must be aware of the Due API. As such the "Agenda" iOS calendar app has integrated the "Due" API, and is able to set a reminder in Due for an event created in Agenda. But it cannot search for existing reminders in Due, or just start Due (or close it) etc.

So in order for an app like QS to exist on IOS, it would need to incorportate the API of every other app that it might want to connect to, and be limited by the fact that a) not every app (in fact I'd say very very few) actually has an API, and b) even where they do, they are often tied down to very specific functionality.

A bit rambly, but I hope this answers your question.

Just got an upvote on this, and re-read, and there is more the the answer than when I first made it. There are a few apps these days that are trying to get closer to what the OP wanted, by effectively collecting and allowing the use of as many of these 3rd party APIs as possible in one place. One such ass is Launch Center which collates APIs for nearly 50 apps, allowing you to do all sorts of actions from a single place - it's no quicksilver replacement, but it is an effective way to get at particular app functions directly rather than having to open the app and navigate to the function manually. - 1 example, is using clipboard contents for all sorts of things - tweet using clipboard contents, new task from clipboard contents etc etc. Very useful.