GitHub user lucidBrot has made a nice and concise list of useful GitHub emoji. This is a copy of that with some of my own additions.

meaning emoji text
info :information_source: :information_source:
note :memo: :memo:
warning :warning: :warning:
tip :penguin: :penguin:
hint :bulb: :bulb:
pro :heavy_plus_sign: :heavy_plus_sign:
con :heavy_minus_sign: :heavy_minus_sign:
error :x: :x:
correct :heavy_check_mark: :heavy_check_mark:
question :question: :question:
resolved question :grey_question: :grey_question:
important :heavy_exclamation_mark: :heavy_exclamation_mark:
wip :construction: :construction:
bug :bug: :bug:
careful :eyes: :eyes:
heart :heart: :heart:
+1 :+1: :+1:
-1 :-1: :-1:
well done :tada: :tada: