Mint.com is a hugely popular website for personal finance and budgeting. The site automatically syncs with your bank account, credit cards and investment accounts to give you an accurate, real-time look at the state of your finances. And now, many of the most useful features of the site -- automatic categorization of transactions, interactive budgets and graphs -- are included in the free Mint app.
For tax purposes, two of the most useful features of the Mint app -- available on iPhone, iPad and Android phones -- are the categorizing and tagging features. Mint does a great job of tracking and categorizing everything that you pay for with your credit card. If you use a business credit card for all business expenses, then those charges will be automatically categorized as such. When it's time to calculate your itemized business deductions, it's all right there.
You can use the app to tag transactions that aren't automatically categorized as tax-deductible. Create specific tags for different tax-deductible expenses: medical expenses, charitable donations, property taxes, etc. When it's time to total up your expenses, you can simply sort by the tags you've created.