February 26 is the deadline for teams to submit offers to their restricted free agents, and also exclusive rights free agents (players with only 1 or 2 years in the league).
April 17 is the deadline for another team to sign a RFA to an offer sheet.
April 24 is the deadline for teams to exercise right of first refusal, ie. match the offer sheet, if their RFAs are signed by another team.
2009 NFL key dates