For 10 minutes on July 18th a corrupted packages file was posted. If you loaded Cydia during that 10 minute period, your packages file became corrupted. This subsequently causes Cydia to crash each time you use it.
Here are ways to fix it for Windows and Mac users:
**Update 1: crash-x has made a small update to the PDF. Please follow instructions carefully. We have confirmed reports that one or more source-repos besides TheBigBoss may be causing your problems. Removing all the lists may help.
Windows users: Download this tool for Windows (.NET required) and unzip the files to a handy location. Run the enclosed executable and connect your device to your PC. Click the button to start the fix. A confirmation screen will appear shortly when it has finished.
Mac or Windows users: This document (PDF) describes how to fix Cydia using DiskAid for Mac or Windows.
Note: This is not a fix for all Cydia issues, but only if you loaded Cydia on July 18th during that 10 minute period and now cannot load Cydia at all.
Thanks to DB42 of OpenHebrew and crash-x for the tools and instructions.Mark Bruce & Kory Lee