Ever want to install a mod, but you can't use winwar 7zip or things like that. If you have, YOUR PROBLEM IS SOLVED :D, if you haven't, this might be useful later. (this only works on windows as far as i can tell) STEPS:

1. Go to your bin folder.
2. Copy and paste your jar.
3. Change the jar to a zip. It will say minecraft.jar, change the jar to "zip" , if you can't see that .jar, go to the control panel, go to folder options, go to view, look for "hide extensions for known type of files" UNCHECK it, apply and you should see it.
4. Extract the zip to a folder.
5. Insert mod files into the FOLDER
6. Go into the folder.
7. Select all (ctrl a).
8. right click, scroll to send to, select zip folder.
9. when i finishes, drag it to the bin folder (out of the extracted one) and delete you first jar (remember the zip can be a backup)
10. Rename your new zip "minecraft.jar" and run minecraft.