Back Taxes & Missing Returns
One of the most common things I help people with is cleaning up years of delinquent filings. (don't worry, it's easier than you think)
I also help people who have large tax debts that they can't afford to pay, with a pretty good success rate.
Tax Audits & Disputes
I handle a lot of tax audits; most are fairly easy to resolve, but some end up in Tax Court.
I've also represented people in IRS criminal investigations.
I have a lot of international clients and thus I deal a lot with tax treaties and other international tax rules.
This includes many of the reporting requirements. (e.g., FBAR, 8938, 3520)
State & Local Taxation
In addition to helping people with IRS problems, I deal a lot with state tax issues, including sales tax and state income tax.