ex officio - Literally, by virtue of one's office. The term refers to the practice under Senate rules that allows the chairman and ranking minority member of a committee to participate in any of the subcommittees of that committee, but generally not to vote.
Congressional Sessions lists all Congresses and corresponding years - from 1789, when the 1st Congress met, through the current Congress.
You can access legislative information, by bill number or key words, from the THOMAS Web site. Information from the present back to the 93rd Congress (1973) is available on THOMAS.
For information on the Senate, Congress, the legislative process and the federal government, the Virtual Reference Desk is a good place to begin.