I know you all probably know this, but for those who don't...

How to get skins without buying Minecraft!

Step 1. Find a skin pack. Using the one provided with minecraft.jar works too.
Step 2. Edit the skin if you want.
Step 3. Move the skin file to the "skins" folder in the World of Minecraft client folder.
Step 4. Enjoy your new skin!

The skin is transferred to bots, so I assume it shows your character with your custom skin to others.