Unrestricted free agents are players with expired contracts who have served four or more seasons. They have the option of signing with any franchise. Undrafted free agents are NFL Draft eligible players who were not picked; they can negotiate and sign with any team. The last time an undrafted free agent started a game was Matt Blanchard of UTEP in 2004. The answer to this question is yes, an NFL player can play for free. But only if he's an unrestricted free agent or if his current club decides not to match another club's offer sheet. In other words, if you're an NFL player and you want to play for a living, you need to get paid. Even though some clubs might be willing to let them go for nothing, the actual practice is very rare.