simplicitypvp.net

Here's a nice tool for switching between your minecraft versions. You need to provide your own minecraft .jar files though. Just download and run as is. Here's the page for it on minecraft forums.
http://www.minecraftforum.net/topic/514 ... -tool-v10/
Here's a download link on mediafire
http://www.mediafire.com/?ycgzi56s9557h3d

I'll try and find older .jars myself and make them available here. Just put your minecraft.jar into your .minecraft/bin folder then run the program and you'll be prompted to add a new version (even though you may be adding and old version) This is just the .jar switcher so everything else in your .minecraft folder will remain the same.

http://www.youtube.com/watch?v=hlGX_9XeDO8
This seems legit. It comes with alpha but i'm not sure those versions will work with the switch since they come with more than just jar files. I havn't tested them all so they may be modded or bad.

Just thought i'd bring this up for the forthcoming update. What you will need to do is get the jar switcher then open it. It will ask you where your .minecraft is the first time you use it. then it will ask if you want to save your jar. Do that then upgrade to 1.3 and run it again and save that one. Then you will be able to switch between them easily.

1.4 save your old jars