The diameter of axons also makes a difference, the thicker the axon, the faster the rate of action potential conduction. This is seen in invertebrates, such as squid. Squid have giant axons (up to 2mm in diameter), which exhibit rapid conduction rates. See Wikipedia 'squid giant axon', in which it says up to 1mm, but I have measured 2mm myself. These are not found in mammals.
Original research on squid giant axon was by Cole & Curtis (The Journal of general physiology, 1939). For more recent research see Hartline & Colman (Curr Biol. 2007 Jan 9;17(1):R29-35.).
Work by Hodgkin, Huxley and Katz (The Journal of physiology, 1952, 116(4): 424–448) using squid giant axons lead to the elucidation of the mechanism of action potential conduction and a Nobel prize. Academic summary here: http://jp.physoc.org/content/538/1/2.full
Wikipedia: see the Goldman equation (Goldman shared the Nobel Prize with Hodgkins and Katz). This is more frequently known as the Goldman-Hodgkin-Katz equation in UK publications.