There is a list of gateways on the Ripple Wiki Gateway List. The only USD ones during the beta are Bitstamp and WeExchange. Of those I'd recommend Bitstamp mostly because the Ripple markets for them are more active (deeper, better prices, more volume) and the transfer fees are smaller (.2% vs .5%). If you're depositing USD to a gateway via bank transfer you could use whichever one has lower bank fees and inside Ripple exchange USD-WeExchange <-> USD-Bitstamp for .7% (the sum of the transfer fees) provided there is sufficient liquidity (many hundred dollars worth last I looked).

If you aren't set on using USD you could consider Ripple Union for CAD, they charge no transfer fee. CAD-RippleUnion can be exchanged for USD-Bitstamp within Ripple at close to the bank's exchange rates for small amounts.