Didn't have MAC address on Wiznet based ethernet shield (cloned)
Ethernet will work as long as two different machine in the LAN don't have the same MAC address, as @Nathan wrote in his answer.

The MAC address is built by two parts, a prefix which are given to manufacturers of network devices and is called OUI, and a suffix that is the node part that manufacturers can use as they wish.

If you want to build an MAC address yourself, use a Locally administered address.  Universally administered and locally administered addresses are distinguished by setting the second-least-significant bit of the most significant byte of the address.
That will make this address to not use any OUI prefix, as that bit is zero on OUI, which minimize the risk of two or more devices have the same MAC address in same LAN. 

If you get the same MAC addresses you will get very strange results if they happen to be in the same LAN, I tell you.  Been there, done that, and it wasn't fun to debug.

Here are a site that looks up MAC addresses (and generate random ones).  But if you want to see which manufacturer have that MAC address, you read it from IEEE and their list.