xdaliclock is a clock program for the X Window System, written by Jamie Zawinski of xscreensaver, xemacs, and Netscape fame. Based on Steve Capps's DaliClock program for the Xerox Alto and original Apple Macintosh, xdaliclock's main feature is that the digits in the clock window 'melt' into the next one as the clock ticks up. The resulting effect is quite pretty, and xdaliclock can make it even more psychedelic by cycling the foreground and background colours over time. If you hold the mouse button down over the window, xdaliclock will display the date instead.
In addition to the X11 version, a version for PalmOS is available. Both versions can be found at the xdaliclock site at http://www.jwz.org/xdaliclock/. The website also contains an animated GIF demonstration of the digit-morphing effect.