The commercially-published U.S. Code Annotated and U.S. Code Service are more up-to-date, but the government-published U.S. Code is still considered the official version and usually the preferred version if you’re citing federal statutes.
Which is why I’m pleased to say that the House of Representatives’ Office of the Law Revision Counsel, which is responsible for the preparation of the U.S. Code, has posted PDFs of all five cumulative annual Supplements to the official 2000 edition of the code.
They have also begun posting PDFs of the official 2006 edition of the code. At present, Titles 1-9 of this new edition are available, and I’m hopeful that the remaining Titles will be posted in the near future.
On a somewhat-related note, a very useful, long-standing feature of the Law Revision Counsel’s web site is a set of tables that shows where recently enacted statutes are expected to be codified.