There are several resources that are useful for tracking various versions of a bill as it moves through the legislative process.
THOMAS - from the Library of Congress, has Bill Tracking Reports for Public Laws (from 1973)
Congress.gov - "Congress.gov is in an initial beta phase with plans to transform the Library of Congress's existing congressional information system into a modern, durable and user-friendly resource. Eventually, it will incorporate all of the information available on THOMAS.gov."
LexisNexis Congressional Bill Tracking (from 1989)
GovTrack.us - a non-commercial, non-partisan, community & open source project. Based on THOMAS data, the House and Senate websites, and related sources, GovTrack allows users to set up monitors, customized email alerts and RSS feeds.
OpenCongress - supported by the Sunlight Foundation and draws heavily on the files available through the GovTrack project. The site also integrates blogs and online news sources.