The actual day that free agents can sign with another team is July 11. However, all 2011-12 contracts expire at 9PM AZ time tonight. Those players without 2012-13 contracts become free agents, to some degree, on July 1. At 9PM tonight, each free agent can be courted by any and every NBA team.
Unrestricted free agents (like Deron Williams and Steve Nash) can agree to contract terms whenever they want, though they can't actually sign on the dotted line until July 11. Most of the best unrestricted free agents will be committed to new teams by then.
Restricted free agents, though, cannot commit to anyone until they can sign an "offer sheet" and those cannot be offered until July 11. So those teams who want to make an offer to Eric Gordon or Nicolas Batum have to wait until July 11 to make their move. The Suns, for example, will likely wait until July 11 before making any free agent moves to fill their shooting guard position, since their top target appears to be Gordon.
Who will sign where? Will anyone agree to contract terms right after the bell rings?