I have used Mindmeister in class with low-level learners to create mind maps of lexis on their iPads, iPhones, and Android Phones, when brainstorming ideas for a new topic. Mindmeister creates a tidy mind map that can be saved and referred to out of class, enabling students to review new vocabulary. Learners can personalise their mind maps or create collaborative maps that they share.
With Cambridge exam classes I have encouraged students to use Mindmeister to organise their ideas when writing plans for their essays, reports, proposals, articles, and letters. We have also created a model mind map for the plans as a collaborative task using an iPad which was later shared amongst all the learners.
Gaming is becoming ubiquitous with language learning, and many enthusiasts are sharing their ideas and reflections on their blogs, like Graham Stanley (https://gamifyingelt.wordpress.com/) and Paul Driver (http://digitaldebris.info/).
Scribblenauts is not a explicitly a language learning app, but you can create items by typing them on the screen, and they appear, so in a sense it is very appropriate as an app to learn and practice English by gamifying the learning.
The idea behind Scribblnauts is that you create a super hero or super villain persona, and use the power of your imagination to create objects that are used to solve puzzles.
Available for iPad, iPhone, iPod Touch, Android Phone and Windows Phone.