UPDATE (9/21/2018): I've also found Super Potato Bruh, a challenging platformer that appears to be in the same vein as Super Meat Boy.

This came up as a question on Stack Overflow a while ago. While Python is noted for its use in many games, here's a list of professional-quality games that use Pygame:

Dangerous High School Girls in Trouble is a 2D RPG by Keith Nemitz that was nominated for a Writers Guild Award, won "Most Innovative Game" by the Casual Games Association, and was also selected as a finalist for the 2008 IndieCade Festival of Independent Games. It is available for purchase off of Steam. Gameplay video

Analogue: A Hate Story is a visual novel-style game that was a 2012 IndieCade Finalist. As of December 2012, Analogue has sold at 40,000 copies. Steam store link (with videos).

Frets on Fire is an open source Guitar Hero clone. Gameplay video

7 Grand Steps is a board-like puzzle game from Keith Nemitz of Mousechief and is a 2013 Independent Games Festival finalist. Trailer video

Metin2 is an Oriental-themed action MMORPG. Trailer video

Galcon 2 is a high-paced space shooter game. Gameplay video

SolarWolf is an updated clone of the 1983 arcade shooter Solar Fox. It is included with several Linux distributions. Download links Gameplay video

División Especial de Detectives is an original puzzle game in Spanish that explores the culture and history of Uruguay. Gameplay video

The Witch's Yarn is an interactive story game from Mousechief. Gameplay video

While not a game, someone has mentioned that the Minecraft editor MCEdit used Pygame. MCEdit tutorial video

Feel free to add any other professional-quality games that have been made using Pygame in the comments section!