Record Keeping
Please don't consider this an exhaustive list, and independently verify everything as rules continually change. If you know of changes or additions, please e-mail them. And, when in doubt, keep the paper.
Houses - Though you do not need to keep records and costs associated with past houses after you sell your current house and buy your next house, if you have ever deferred the gain on the sale of a house and rolled it into your current one, you'll need the home sales records to compute the taxable gain when you sell (though not likely to be challenged if the house sells for less than $500,000.). Keep in mind that single people can exempt only $250,000 from tax, so this makes it more necessary to save these records if you are, say, divorced or widowed and end up with the house. And some advise you to keep records anyway, like documents showing how much you paid for the house, the cost of any major home improvements over the years, and any depreciation.
Traditional IRA - Keep a record of any non-deductible contributions so that you are not taxed a second time at withdrawal.
Roth IRA - Keep records of contributions until age 59.5, because holders may make an early withdrawal of contributions without tax penalty.
Education IRA - Keep track of the account in general as well as the age of the beneficiary (no contributions after the beneficiary turns 18, and balances must be used before the benficiary turns 30, though unused balances can be rolled over to another child) and that any disbursements went to qualified higher education expenses.
Capital Gains - Keep records of mutual funds, stock purchases and sales, dividend reinvestments, etc. and information about the length of time you have owned the assets. Some brokerage firms give a year-end statement that can be kept rather than the detail.