iPhone: If you take and share a lot of screenshots on your iPhone, you know the value of an annotation tool. Blink is one that's easy and quick to use.
Blink is all about annotating, highlighting, or adding giant emoji to a screenshot. Just snap a screenshot and you can get to editing it immediately in Blink. You can make memes or draw on screenshots too, if that's more your thing.
There's a whole social network component as well, but you don't really need to engage with that if you don't want to. If something like Skitch isn't goofy enough for you, Blink's worth a look.
Blink (Free) [iTunes App Store]