Edit: Unfortunately, the images originally included in this post are gone, because of hosting problems in late 2009. My apologies.
Tom Cornall left a note on the Software page about a slender (some might call it anorexic) little program called lalcal, which does only about two things: Shows the date and/or time in your Openbox dock, and a calendar when you click on it.
There are probably a thousand little applications that can do this, and I have tried a lot of them before. dockapps.org is always a good place to look, if you want desktop doodads that do similar stuff. I mentioned some here, about a year ago. The reason I don’t use them is that they usually look obtrusive or out-of-place, and they kind of interfere with the way I arrange the windows.
On the other hand, I’m hooked on lalcal now, partly because it’s very unobtrusive, but also because it requires about 12 seconds of your life to get it started. Here it is in action in Crux, if you can find it.
And here it is with the calendar expanded, in case you couldn’t find it in the first photo.
And that’s about everything it does. No fat dependencies, no major hassles to compile, customizable from the command line or from your .Xdefaults file, and handles anything strftime can do. Make and move to where you want to trigger it from; I put mine in my home directory as .lalcal, and start it with the .xinitrc file. Light, simple, easy … magnificent. :D