The following websites are various Tax Code resources which I have found to be helpful.

Internal Revenue Code - Fourmilab

If you are to use any search engine and conduct a search on the phrase "tax code", this website is the first result on the list which provides the United States Tax Code. It is a version of the Tax Code which provides hotlinks to sections referenced within 26 USC and is created by John Walker, who is the co-author of AutoCAD. However, as you might notice in the preview descriptive text of this website in your search results, the Tax Code which is provided is current as of 2004 - and has not been revised since. When I first started the OneLine Tax Code, I did use this website as the source of 26 USC until it became evident that it was far out of date. In light of this, I emphatically encourage you to not go to this website (which I why I don't have the address hotlinked here) until the code proivded there undergoes necessary revision. Recreation of the Tax Code is no small feat so I in no way intend for this request to belittle or diminish his accomplishment; without knowing what else was available on the Web in respect of the Tax Code at 2004 it might well have been the case that his version of the Tax Code was the only version online which took advantage of HTML at the time. I only encourage you to use other resources for Tax Code research because they will more likely be more current. Despite all of this, he does have some sage advice regarding reference to the Tax Code and his website.

Cornell University Law School U.S. Code Collection
Title 26 - Internal Revenue Code

This is the main resource from which I have created the OneLine Tax Code. The complete text of 26 USC is provided here and displayed in a more readable format, complete with proper links given for references to other provisions within 26 USC as well as anywhere else within the United States code, in addition to certain Public Laws. It provides an RSS feed so that you can see new additions and adjustments to the code as soon as they have made them available.

U.S. House of Representatives Downloadable U.S. Code
United States Code: Main Page

This is the most current and official version of the United States code made available on the Internet. With these two websites you will have the capability to find the most recent versions of 26 USC and provisions therein as they are published by Congress. The Downloadable US Code site allows you to download individual chapters or the entirety of 26 USC itself, which is a 26 MB file of pure text (as of 2009). The Code you will see here is all in pure text format, which means there are no hotlinks in any document and the formatting of the content is minimal.

The Internal Revenue Code of 1939
The Revised Statutes of 1874 (relating to internal revenue)

Statutes at Large Links: U.S. Congressional Documents

If you wish to research older laws regarding the United States, these links should prove to be helpful.

Rules and abbreviations. Table of Contents - 26 USC The OneLine Tax Code.
The above links will open their pages in new browser windows.
Home Page.