Postal is a DBPF3 package editor. That means that it can open up Sims 3 package files, let you poke at their innards, change things around, then save them back into new package files.

Can I run it?
Postal is built on top of Java 6, so if you can run Java 6 then the answer is "yes". That includes Windows XP, Vista and Mac OSX. If you can run the Sims 3, you should be able to run this.
There is one caveat though. Because I'm running Windows, I cannot test well on Mac. Please let me know if you experience any issues.

What can it do?
Postal can navigate all kinds of Sims 3 package files, and parse/display/write many resources, including OBJDs, OBJKs, STBLs, XMLs, CSTRs, GEOMs, NMAPs, DDSs, PNGs, VPXYs, Texture Compositors and the various text-based formats. As time goes on, the number of resource types it can edit is growing.

Postal also has a filetable which keeps track of all the resources that can be used in game. You can use this to search for resources by type, group, or instance, wherever they come from. Postal has a scripting console, and a plugin architecture to allow you to add your own functionality to the application.

The Postal API
The Postal Editor is built on top of the "Postal API". This is a library which speeds up the process of writing custom tools which can read from and write to package files and the resource types inside them. The API is available under a BSD license, which essentially means you can use it however you like in your own tools as long as you acknowledge the original creator and point to a place where the original source can be downloaded. (The full Postal GUI is available under a GPL license)

The Postal API has already been used in other tools, including The Sims 3 Mod Generator and Merlin . There is some documentation on how to use it to build your own applications and how to extend the Postal environment attached.

INSTALLATION INSTRUCTIONS
- Go to http://www.java.com/en/download/manual.jsp and install the up-to-date version of Java for your operating system (if you don't already have it)
- Download the latest version of the Postal installer from http://sourceforge.net/projects/postal-sims3
- Double click on the file you have downloaded, and follow the Wizard. Make sure you put in the correct path to your Sims 3 installation when it prompts you!

* Mac OSX users, please see this post regarding running Java 6

Additional Credits:
Most of the behaviour here is based directly off the wiki pages describing file formats and resource structures. Thanks to Rick, Karybdis, Delphy, WesHowe, Inge, Peter, Atavera, Pescado and the many others who have contributed their knowledge to that collective effort.

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1075 people have said thanks to Echo for this download.

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Basic Download and Install Instructions:

1. Download: Click the download link to save the .rar or .zip file(s) to your computer.

2. Extract: Use WinRAR (Windows) or Stuffit (Mac) to extract the .package file(s) from the .rar or .zip file(s).

3. Go to http://www.java.com/en/download/manual.jsp and install the up-to-date version of Java for your operating system (if you don't already have it)

4. Download the latest version of the Postal installer from http://sourceforge.net/projects/postal-sims3

5. Double click on the Postal-installer file you downloaded from SourceForge, and follow the Wizard. Make sure you put in the correct path to your Sims 3 installation when it prompts you!

Need more information?

• For a full, complete guide to downloading complete with pictures and more information, see: Game Help: Downloading for Fracking Idiots.
  
  
• Custom content not showing up in the game? See: Game Help: Getting Custom Content to Show Up.
  
• Anyone can use both .rar and .zip files easily! On Windows, use WinRAR. On Mac, use Stuffit.