Rainlender and Rainmeter

I'm going to do these two together, because, well, they're by the same author and do similar functions. Rainlender is a desktop calender/reminder system. Rainmeter is a desktop system monitoring tool (although it can do much more than just monitor CPU usage etc). The main selling point of these two programs are their visuals really. If you've seen the widgets in Mac OSX then that's basically what you're getting with this sort of thing. A nice looking mini-program that sits on your desktop and provides simple functionality. For example, in rainmeter you might have a small widget that grabs the weather reports from the net and displays the temperature and weather conditions in a nicely formatted graphical way.

They're both very customisable, and in my experience, reasonably light on resource usage...but I found both very difficult to customise. They're very, very customisable - but you're going to need to put some work into getting to know the way they work. This is no point and click task unless you're happy using other people's skins.

Personally, I think Konfabulator is a much easier program to use and replicates the functionality of Mac OSX's widgets more accurately (it also seems to have a bigger community centered around it - so more widgets are being developed by other people for this). Yahoo bought the company out a few weeks ago and are now offering the software for free (you had to pay for it previously). Although it isn't open-source, Konfabulator seemed to be much more useful in a practical sense - but far less customisable visually. I guess it comes down to what you want.

If you've never seen either of these before, then I suggest you take a look around the sites linked below. There's some very, very nice looking stuff being developed for them.

Rainy's (Rainmeter and Rainlendar) Homepage
Konfabulator Homepage (free but NOT open-source)