"Teaching Fellowships for Aspiring Law Professors":

Paul Caron (TaxProf) provides a long list of options (certainly not limited to tax).

UPDATE: Turns out that Harvard has a new fellowship -- and a very attractive (though I'm sure very competitive) one at that. It's in the Health Law Policy, Biotechnology and Bioethics program; they're planning to have 8 fellowships, which will be for staggered terms of 2 years, $60,000/year, with no teaching obligations, just time and money that lets young scholars write the articles that they'll need to get their careers started. Nice work, if you can get it.