These are so fun it's ridiculous, seriously.
The creator of the desktop ponies applications has written to inform everypony that the app has gotten a massive update- lots of new characters, including a Trixie with fireworks! Go download it and try it out, you won't be disappointed.
Their website is here, and the Ponychan thread is here.
Specifically, the programmer in charge points us to this and this post in the thread.