IRS.gov You can search assorted topics in all IRS publications or specifically in the Internal Revenue Bulletin, Internal Revenue Manual, Tax Regulations -- for example Regs 1.46-3(d)(2).
The name of the more-specialized Tax Statistics database says it all. This site (produced by the IRS) has some interesting data compiled from tax returns filed by individuals, corporations, charities and so forth.
Click here to see a screenshot of the At-a-glance screen disclosing for the most recent tax year when data was tabulated and analyzed these kinds of statistics:
Please note: For tax forms themselves, the main IRS link at the top of this box is the place to go.