You absolutely must use salt. If you do not the passwords are easily reversed using rainbow tables. A very simple salt that works fairly well, have the server send a random salt with the page. MD5 the password, then MD5 the first hash and the salt together. On the server side you can store the MD5 password, and run a quick MD5 on the stored hash and the salt from the page; compare and authenticate appropriately. This sort of salt is known as a nonce. You should also add a long realm salt to both the original password and the one stored on the server.
So a "good" way to go (short of SSL and the others mentioned above) is:
A = MD5 ( Password + Realm)
B = MD5 ( A + Nonce)
Send B over the wire. Keep A stored on the server. When setting A in the first place use a simple reversible encryption like ROT or XOR. Personally I would consider anything less than this irresponsible because so many users foolishly use the exact same password for darn near everything (even Jeff - site founder - has done this).