Essential Software for Thumbdrives?

MaufTarkie asks: "My manager just handed me a 256 megabyte thumbdrive. I already know about UnixKit for Windows and the WEP Key Generator Utility for Wi-Fi Networks, but I'm interesting in hearing what other people are putting on their drives. My drive will potentially touch a range of systems (Windows, Unix, and MacOS X), so any software is fair game. What are some essential tools I can (should) put on this?"