If you mean commission-free -- Schwab lets you trade their Schwab ETFs commission-free, with no minimum holding period. Here are some I had from notes a while back:
SCHF SCHC SCHE SCHZ SCHP SCHO SCHR SCHB SCHX SCHG SCHV SCHD SCHM SCHA SCHH
Etrade also offers around 30 I think from different sources, but they charge the commission if held less than thirty days.