Webdesigner Depot is toasting Photoshop on it's 20th anniversary with a wonderful trip down memory lane that traces each of the many versions leading up to the current CS4.
Greg can fondly remember version 2.0 that came on a floppy disk and ran on a Mac with an 80MB hard drive.
For me the journey starts in 1998 with Photoshop 5 and 5.5 which was released just a year later and included the new "Save for Web" feature. This all coincided with my first year at Drexel University.
I had never owned my own computer until that time and Drexel's policy required all students to have one of their own. I jumped in headfirst and stumbled through many clumsy attempts at webdesign, inescapably leading to Photoshop, image slices, and the "Save for Web" function.
"Save for Web" was also part of ImageReady, a companion program to Photoshop that has since been absorbed by Photoshop itself. At that time it never would have occurred to me that my new passions would eventually lead me back to my childhood love of photography.
One fun thing that Webdesigner Depot does not cover is that each of the more recent version of Photoshop have shipped with the "About" screen Adobe used in-house during the development of that version. This easter egg can be seen if you hold Command+Option+Shift while clicking on "About Photoshop" in the menu (substitute Control for Command on a Windows machine). My favorite was always Venus in Furs from Photoshop 6. A short history of these hidden splash screens can be found here. The current version, Photoshop CS4, has the above Stonehenge hidden screen.