Note: I started this page because the subscription question came up when we weeded the Public Health Reports. I don't know what is available or if the GPO has a list of such resources. But, I am sure it will become more of an issue with as more government information becomes available electronically. So this page could also be titled "Charged for the Electronic Version, Free for the Paper Version."
There are some agencies within the U.S. government, like GPO, that are required to recover some of their expenses through fees. The National Technical Information Service is another which is why citizens are charged for obtaining copies of their reports. Recently through the DARTS program, Federal Depositories have been given usernames and passwords to access the NTIS database. ( link )
Items Requiring Online Subscription:
Climatological Data: (C 55.286: through C 55.289: ) The electronic version is available on line at the National Climatic Data Center http://www.ncdc.noaa.gov/oa/ncdc.html
NTIS: "The National Technical Information Service serves as the largest central resource for government-funded scientific, technical, engineering, and business related information available today. For more than 60 years NTIS has assured businesses, universities, and the public timely access to approximately 3 million publications covering over 350 subject areas." (From the NTIS website 5/07/09, http://www.ntis.gov/about/index.aspx)
Public Health Reports (HE 20.30:) Depositories already receive the paper copy of Public Health Reports if they select it. I'm not sure one can get free access as a depository to the electronic version.
Stat-USA Depository libraries get free access for one user at a time, but must get login information from the agency.