Randy Moss is the Pats' top candidate to receive the franchise tag. Asante Samuel got it last year, but under the condition that he wouldn't be franchised again. Randy Moss is an unrestricted free agent, and after setting the NFL record for touchdown receptions, is the top free agent WR out there.
A franchise tag would give Moss a 2008 salary of $7.84 million, which is the average salary of the top 5 WRs in the League. This would be far below the potential salaries he could garner on the open market.
Other teams would still be able to make offers to Moss. But if Moss signed with them, it would cost them two first round draft choices. That price is a bit too steep, which is why franchising players is such an ironclad way to keep them on your team cheaply.
We can all agree that Moss was a huge part of the Patriots' record-breaking offense. Although he didn't have big numbers in the post-season, that was more of a product of Tom Brady's play, and not Moss. He is the ultimate receiving threat. He's fast, tall, strong, with great hands.