There are a few ways to do that or something similar:
1. On
http://www.7is7.com/otto/countdclock.html you could generate a
clock using the the "Generate Raw URL" or "Generate W3C URL" options
and then create a bookmark for that URL.
2. Or after generating your clock you could right-click on the clock
and select "This Frame" -> "Show Only This Frame" (In Firefox). Then
bookmark that page.
3. Ot go to
http://www.7is7.com/otto/countdbookmarklet.html which is a
special bookmarklet generator and safe the bookmarklet link (or drag
it to you bookmark folder). This should actually give you a clock in a
small popup window.
Hope this helps :-)