Particle accelerators such as that at CERN accelerate particles to high speeds before colliding them and measuring the particles given off.  However it is almost always protons that are collided, or ions or neutrons. Why doesn't anyone ever collide electrons?  Even if you did collide electrons, what sort of things would you see given off?

EDIT:

thanks for the answer, i'm also interested in what sort of particles are formed from electron collisions and how much energy is needed to form these particles.