Hi there battlestar, and welcome to ECN.
1. Assuming that we're talking about relatively low-voltages, the typical way that such a task is carried in modern electronics is by using an operational amplifier (frequently called an op-amp). These come in integrated circuit form, and require just the addition of a few components to set up the correct conditions for whatever task is required.
A simple adder circuit is fairly easy to achieve, and in fact was one of the earliest applications of the op-amp in analog computers.
Have a look here for some information about the op-amp and some of its uses:
Check the page Current Summing Amplifier
for your specific task.
2. Assuming we're talking about a small DC motor, a bridge circuit can be employed for this task. The exact arrangement will depend very much on the signal levels and power required for the motor, but you might like to explore some of the links here for ideas: http://www.epanorama.net/links/motorcontrol.html
[This message has been edited by pauluk (edited 02-20-2004).]