This easy to use add-on for Blender adds a new tab to your Tool Shelf called Zero Selected Vertices. This tab contains three buttons, Zero X, Zero Y, and Zero Z, which will set the vertices you select to 0 along each of their respective transform axes. See the video above for a demonstration.

To install the add-on, download the file and open Blender. Go to File > User Preferences > Add-ons, and click Install Add-on from File. Navigate to the directory that you downloaded the add-on file to, and select it. Once it is loaded, click the checkbox to activate the add-on.

I find this add-on very useful for making sure the center edgeloop of a retopologized character is zeroed out so I can mirror it easily.

(Please do not redistribute or resell. If you like this add-on, consider tipping the creator with the "Pay what you want" feature.)