The USL Premier Development League is the top level of amateur competition in the USA and Canada, and the unofficial fourth tier of soccer, behind MLS, USL-1, and USL-2.
Here's a quick blurb about the PDL from the USL website:
"Players have the ability to compete in the PDL throughout the summer months in a professionalized setting while maintaining their collegiate eligibility. The league is the proving ground for players that aspire to reach the highest-levels of the game, while providing affordable family entertainment within their city."
These days, quite a few PDL teams serve as farm clubs for USL and MLS teams. The Colorado Rapids U23's, Chicago Fire Premier, Cary RailHawks U23's, Atlanta Silverbacks U23's, and Richmond Kickers Future are all in the PDL today, and many MLS teams are rumored to be starting up U23 PDL teams in the future.
A large number of current MLS players, including standouts Brian Ching, Brad Guzan, Chris Rolfe, Michael Parkhurst, and Sacha Kljestan have played in the league.
Quite a few PDL players have eventually made the big jump to Europe as well.
Here is a list of the developmental teams these players played for and where they are in Europe today.
[edit: added Jeremiah White]
Kamani Hill - San Fernando Valley Quakes - Orange County Blue Star - VFL Wolfsburg
Sal Zizzo - Orange County Blue Star - Hannover 96
Colin Burns - Delaware Dynasty - Ljungskile SK
Pat Noonan - Michigan Bucks - Aalesunds FK
Jay DeMerit - Chicago Fire Premier - Watford
Tighe Dombrowski - Chicago Fire Premier - IK Sirius FK
Matt Pickens - Chicago Fire Premier - QPR
Jonathan Spector - Chicago Fire Premier - West Ham United
Joseph Ngwenya - Cape Cod Crusaders - SK Austria Kärnten
Troy Perkins - Cape Cod Crusaders - Vålerenga I.F.
Ethan Zohn - Cape Cod Crusaders - Survivor Africa Winner
(Sorry, I don't even like that show, but I couldn't resist)
Olivier Occean - Vermont Voltage - Lillestrom SK
Charlie Davies - Westchester Flames - Hammarby IF
Heath Pearce - Bradenton Academics - Hansa Rostock
Stern John - Carolina Dynamo - Southampton
Jeremiah White - Ocean City Barons - AGF Aarhus