Asteroids in langrangian Points 4 & 5
There is one known earth Trojan - http://en.wikipedia.org/wiki/Earth_trojan

The reason L1, L2 and L3 aren't stable is covered here:   https://physics.stackexchange.com/questions/36092/why-are-l4-and-l5-lagrangian-points-stable

The reason the earth has so few L4 and L5 asteroids is cause other planets like Jupiter and Venus tend to destabilize the orbits - same reason why  Jupiter has so many.