toastmasters - especially the improv "table topics" activities - has been indescribably valuable for me.

It helped me go from being too shy to give a presentation in front of one person to giving talks to hundreds of strangers at a time.

Pro-tip: you can attend the first one or two meetings of each toastmasters group for free so if you're a broke student like I was at the time, just rotate the groups since there are several in each city. Also universities often have great toastmasters clubs.