I'm pleased with my HSA account that I have with HSABank.com
Even though they charge a monthly fee for a brokerage account AND a low-balance charge if your bank savings account falls below $3,000 cash (although they recently switched clearing agents/brokers, so harrisdirect may not charge as much), I am still able to clear about 6 1/2% to 7% off of my investments after all of the fees...and that's just with two years' worth of contributions. Can't wait for 3 years from now, when I'll have considerably more chugging away at the yield
HSABank seemed to have the lowest charges for both starting out and long-term, and allows you to open up a brokerage account and invest in literally anything/everything. I haven't done a survey in a while among the massive (and constantly growing) list at hsainsider.com, but check them out and see which providers look like the best. Remember to include ALL fees in your calculations, which would really throw a wrench in the first few years when your returns might total a whopping $250-$500.