iPhone: Annotable is one of the best image annotation tools around, especially after Evernote abandoned Skitch a couple of years ago. Now, it's been updated with a slew of new features, making it much more versatile.
The new customisation options allow you to change which tools are shown on screen and change the colours of any tool. You can also add in new fonts, styles, stamps, labels, pointers, and more. The new pointer styles are an especially nice addition for those who weren't happy with the boring old lines.
Like the previous version, while Annotable is free, a lot of these new tools are paid in-app purchases, which makes for a bit of a confusing toolkit. Still, as far as a la carte pricing goes, it makes it so you can buy one new tool for just a couple bucks and then skip over anything you don't use.
Annotable (Free/IAP) [iTunes App Store]