The Center for Responsive Politics has a new database where you can track the number of lobbyists that reported lobbying on a piece of legislation back to 2005. The database is searchable by a bill's name, description, or number.

OpenSecrets.org's Lobbying Database provides information about the clients that paid for the work and the lawmakers sponsoring the bills. Disclosure forms ask for the lobbying issues worked on during the period, and some filers name bills by number and title while others just provide vague descriptions. The database will be updated after the first lobbying reports of 2009 are due on April 20. According to the database, "of the 10 most lobbied-on bills in the 110th Congress (2007-2008), seven were appropriations bills."