Topology on fundamental group so that its a topological group

Question

Let $(X,x_0)$ be a pointed topological space.Let $ \pi _1(X,x_0)$ denote the fundamental group of $X$ based at $x_0$.What are 'good' topologies one can put on $\pi_1(X,x_0)$ so that $\pi_1(X,x_0)$ is a topological group?