Unknown to some Americans, our neighbors to the north have their own professional league of pigskin-carrying warriors.
The Canadian Football League has produced some premier talent throughout the years, some of which went on to the NFL.  Here is a list of the five best NFL players in recent memory who spent some time in the CFL:

5. Ricky Williams
Ricky Williams rushed for more than 1000 yards in four straight seasons from 2000 to 2003.  In 2002, he rushed for a career-high 1853 yards and 16 TDs.  After his early retirement and subsequent suspension, Ricky Williams spent the 2006 season with the Toronto Argonauts.  He got injured after playing 11 games in the CFL and ended up back in the NFL the following season.  After that, Williams never really regained the form of his earlier career, but managed to put up 1,121 yards and 11 TDs in 2009.  His numbers since have severely dipped and he has recently taken a backseat to Ronnie Brown and Ray Rice.  Before retiring for good last week, Williams became the 26th player ever to run for 10,000 yards in his last regular season game this year against the Cincinnati Bengals.
4. Jeff Garcia
Jeff Garcia started his career in the CFL as Doug Flutie’s backup for the Calgary Stampeders.  In 1998 he lead the Hamilton Tiger-Cats to a Grey Cup, which is the CFL’s championship game.  He signed with the San Francisco 49ers in 1999.  In his first full season as a starter with the 49ers he threw for an impressive 4,278 yards, 31 TD, and 10 INT and rushed for 414 yards and 4 TDs.  In 2001, Garcia became part of an elite group of nine quarterbacks who have thrown for back-to-back 30 TD seasons.
3. Mike Vanderjagt
The most accurate kicker in NFL history, Mike Vanderjagt, got his start with his hometown Toronto Argonauts.  Vanderjagt was a two-time Grey Cup champion before he got snapped up by the Indianapolis Colts in 1998.  During his NFL career, Vanderjagt kicked 230 field goals out of 266 attempts (86.5%).  Unfortunately for Vanderjagt, he will always be remembered for missing the most important kick of his career.  As time winded down in a 2006 divisional playoff game, Vanderjagt shanked the game-tying field goal and sent the Pittsburgh Steelers to the AFC Championship game and beyond.
2. Doug Flutie
Doug Flutie had plenty of memorable moments in his career: from his famous Hail Mary in his Boston College days to his drop-kicked field goal in his last game for the New England Patriots.  The 5'10” quarterback won three Grey Cups in the CFL before getting signed by the Buffalo Bills.  He led the Bills to their last Playoff appearance only to be sat by coach Wade Philips in the "Music City Miracle" game in favor of Rob Johnson.  Towards the end of his career, the ever-mobile Flutie became the first 40-year-old player to rush for two touchdowns in a game.
1.Warren Moon
Warren Moon started his career playing for the Edmonton Eskimos, where he won five consecutive Grey Cups before ascending to the NFL.  During his career, Moon threw for 49,325 yards and 291 TDs.  He also rushed for a career 1,736 yards and 22 TDs.  Moon set NFL records for attempts, completions and TDs, which were eventually broken by Brett Favre.  He was selected to the pro-bowl nine times and was awarded NFL MVP in 1990.  Warren Moon became the best quarterback to never win a Super Bowl whose first name was not Dan or Jim. 
Honorable mentions: Cookie Gilchrist, Joe Theisman, Harald Hasselbach...