HSAbank is one that's widely used, and recommended. With HSAbank, you can create a TD-Ameritrade account for the investment portion of your HSA fund.
The HSA I have with them imposes a $3/month fee for cash balances below $3K, so I keep $3,003 at HSAbank, and transfer all amounts over $3K (i.e. the monthly paycheck deposits) to the Ameritrade account.