April 15th is not only the due date for submitting your tax forms, but it is also the deadline for teams submitting offers for Restricted Free Agents (RFA's).As of this date, any RFA still unsigned must either sign their current team's tender offer sheet to remain with their team or wait to see if they will be dealt in a trade.
There are still a few RFA's on the market, including the Baltimore Ravens own left tackle, Jared Gaither, who has been given a first round tender, meaning if another team wants to sign him, they would have to give the Ravens their first round draft pick. However, teams have the option of accepting a lesser "offer" from another team and working out a deal to trade that player or "allow" the other team to sign them. While Gaither might be one of, if not the most heralded RFA still on the market, he is not the only one, but expect most of them to ultimately sign their current team's tender and report to Training Camp at some point prior to the season beginning.
Most teams at this point are waiting for the draft to either consider draft day trades or then will fill out their roster with the remaining veterans who are either Unrestricted FreeAgents (UFA's) or trading for one of the previous RFA's out there.