Since I began playing Scrabble as a child it has been clear that knowing the two letter words is vital to game strategy. And while I've seen lists of the two letter words before I've always wanted the list organized in a specific way, namely, laid out by letter with all the words that start and end with that letter. I don't know if at some point I saw this somewhere or if it's just been something in my head. Either way I couldn't find the letters charted the way I wanted so I took it as a good opportunity to mess around with Rapahel.js. Meet two letter word chart. I made sure it formats pretty well on iOS devices (and used mobile Safari's web app feature so you can "Add to Home Screen" to make the site act like an app without the Safari chrome). 
Oh and I've been playing a lot of Words With Friends with my Grandma so it you can switch to WWF's (mystifyingly different) two letter word list as well.