Bloomberg Law
Bloomberg Tax has links to ALL SORTS of regulatory materials.
Hein Online
Hein is an online publisher much like Westlaw or Lexis - the different being their absolute superiority for historical materials. Did you want to see the Compilation of Decisions Rendered by the Commissioner of Internal Revenue under the War-Revenue Act of June 13, 1898? Hein's got you.
They have such a huge body of agency materials that listing them out would be... not a good use of our time together.
Link to a list of all of Hein's Department of the Treasury materials.
Link to all of Hein's Internal Revenue Service materials (which is much shorter than the Treasury Department materials).
Lexis
In addition to its federal materials, Lexis has a pretty good list of state materials (like state Board of Tax Appeals decisions).
Westlaw
Westlaw also has a fair collection of state materials, too, alongside all of the expected federal works.
Abbreviations
Rev.Rul. = Revenue Rulings
TAMs = Technical Advice Memoranda
PLRs = Private Letter Rulings
Citations
ALWD
Bluebook