When Apple moved from Mac OS 9 to Mac OS X in the 2001 timeframe, users lost a feature known as the "Application Switcher", where you could go to the top-right corner of the screen and select open apps, and move between them. Also - the application you would be using would be represented by its name and icon. While it could be argued the "Command-Tab" functionality replaced this to some degree, to me, it's not as flexible as X-Assist, which does all of this and more, including listing recent applications and gaining one-click access to control panels.
Unlike most pieces of software, which make incremental point upgrades on a semi-regular basis, X-Assist met my needs right away, and hasn't needed a new version for three years - last being updated in November of 2003. A lot has happened since then, even as Apple moved from Mac OS X 10.2 to 10.3, 10.4 and is now previewing 10.5, but X-Assist continues to be an integral part of my productivity, and the program has integrated seamlessly with each new generation of the OS.
While Steve Jobs and the Apple team have done a fantastic job introducing greater simplicity to the Macintosh over the years, the wholesale elimination of functionality isn't always a good thing. Lucky for us, independent software developers are often there to help to fill in the rare gaps.