MyEmotes lets you use custom emotes easily. It's the successor to my older addon, Emotomania.
Examples: /grats now works in exactly the same way as /congratulations (why didn't it before? Well, it does now), and you have some other fun ones created using the /e emote, like /r1e (raising one eyebrow), /r2e (raising both eyebrows), /envy, /sulk, /frustrated, and so on. There are tons of preset ones already (have a look in defaults.lua if you're curious).
It's in a feature-incomplete alpha phase at the moment, and like its predecessor Emotomania, it doesn't have a GUI yet, but I use it and it works well already. A GUI will come in future versions to make your own easily.
Note: Due to an intended game mechanic restricting cross-faction communication to built-in emotes only, custom emotes as done with MyEmotes will not work cross-faction (the other faction will simply see “Name makes some strange gestures.” just as it would if you used /e manually).