Married persons who file separately are barred from claiming certain tax credits, such as the adoption credit or education credits. However, there is no such bar for the low-income housing credit.
A retirement account to which up to $4,000 (or $5,000 if you are 50 or over) may be contributed for 2007, but deductions for the contribution are restricted if you are covered by a company retirement plan. Earnings accumulate tax free.