Rainbow Dash does the legendary Sonic Rainboom in SFM.  And more spiffy tricks.

Link for an HD video download and the SFM resources used to make it:
http://www.dos486.com/pony/rainboom

Song: Double Loyalty by justcross7
https://www.youtube.com/watch?v=UpEVVaI4Flk

Song: earth pony magic by sci
https://www.youtube.com/watch?v=2Y0qhOTdo10

Cloud vectors by GuruGrendo
http://gurugrendo.deviantart.com/gallery/34456458

Pony models by Poninnahka (as if anyone doesn't know that, heh)
Source Filmmaker (c) Valve
Voice clips by Ashleigh Ball, Andrea Libman
My Little Pony: Friendship is Magic and all voices and characters are owned by Hasbro and DHX Media.