What can I use to hang a frameless mirror?
At Home Depot (or Lowes) you should be able to find these Spring Loaded Mirror Mounting Clips made specially for hanging a mirror.  They are no where near as big as a j channel and the top two have a small spring inside of them so they kind of grasp the mirror (and snugly fit on the glass).  

You put the two without the springs in the bottom and the mirror just sits in them.  Two more (with the springs) added to the top prevent the mirror from falling forward.  Of course it would be best if you can get them into a beam in your wall.  And you will barely notice the clips on the mirror once it is in place.