A loadbalancer is used in many MySQL setups. Loadbalancing can be
done with an hardware loadbalancer or with software. Software
solutions tend to be a fair bit cheaper.
Loadbalancing is often implemented as an extra layer: The application connects to the loadbalancer via the network and the loadbalancer connects to the database via the network. The loadbalancing can be implemented on the application machines to get rid of the extra layer, this is better for latency (each layer adds latency) and as each application server has it's own loadbalancer it can also remove bottlenecks. The drawback are that you'll lose a single point of control and …