TClock Lite is a vastly improved replacement for the standard Windows tray clock. It provides all the benefits of the original clock as well as many enhancements and a number of useful additional features. Furthermore it achieves this with a tiny footprint and doesn't even require installation. Given the tiny size of the program, the power of this program is most impressive. Less impressive is the help file which is brief and not particularly helpful. However the program is so well laid-out and simple to use that it's not really an issue. Note that you might get a warning from your security software that TClock is potential spyware. It's not. It's just picking up the fact that it modifies explorer.exe to display the clock. Note too that the 2nd link below is to the Japanese author's site. You'll see some strange characters displayed but the "donwload" (sic) link is clear enough.
If you want an alternative try the classic TClockEx that offers similar functionality to TClock Lite. It hasn't been updated for a while but appears to run OK on Windows XP systems.
Another alternative that is still being actively developed try AlfaClock which offers a little more functionality at the cost of some additional complexity.
LClock is may your last, but by not means least, recommendation. It pops up its own calendar with a single click on the tray clock. And it's not just a static calendar: you can use it for reminders and to-dos. It all works marvelously well and as a bonus you get a clock display that looks like the one in Vista. LClock is based on TClock lite but unlike TClock Lite, it doesn't allow a simultaneous time and date display. Perfection is so elusive
TClock Lite
Website: http://homepage1.nifty.com/kazubon/tclocklight/index.html
Download page: http://homepage1.nifty.com/kazubon/tclocklight/index.html
Author: Kazuto Sato
Current version: tclocklight-040702-3
Version date: 8/9/2004
License: Free
Download file size: 373KB
Operating systems supported: Windows 98-XP
Additional software required: None
64 bit version available: No
Portable version available: No installation required
Non-English languages supported: Many; Available via downloadable language packs
Other relevant information: On website.
TClockEx
Website: http://www.rcis.co.za/dale/tclockex/index.htm
Download page: http://www.rcis.co.za/dale/tclockex/download.htm
Author: Dale Nurden
Current version: 1.4.2
Version date: 9/3/2000
License: Free
Download file size: 467,894 bytes or 194,464 bytes zipped file
Operating systems supported: Windows 95/98-XP
Additional software required: None
64 bit version available: No
Portable version available: No installation required
Other relevant information: On website
AlfaClock
Website: http://www.alfasoftweb.com/alfaclock/
Download page: http://www.download.com/AlfaClock-Free/3000-2350_4-10433409.html
Author: AlfaClock.com
Current version: 1.99
Version date: 2006
License: Free
Download file size: 2.09 MB
Operating systems supported: Windows 9x-XP
64 bit version available: No
Portable version available: No
LClock
Website: http://fileforum.betanews.com/detail/1092761816/1
Download page: http://fileforum.betanews.com/detail/1092761816/1
Author: Ying Hang
Current version: 1.62b
Version date: 19/9/2004
License: Free
Download file size: 186kb
Operating systems supported: Windows 2003/XP
64 bit version available: No