Huckleberry Finn Hostel
In two old town houses, this independent hostel is basic, but it's a friendly gathering spot with a piano in the lounge/kitchen, and free lockers. Its Soulard location is ideal. Be sure to reserve ahead.
Just about every midrange and upscale chain has a hotel near the Gateway Arch in downtown. Indie cheapies are thin on the ground in interesting areas but you'll find plenty near the airport and you can ride the MetroLink light-rail into the city. Upscale Clayton on I-170 exit 1F also has rail access and a cluster of chains.