I had recently put together new computer and one of the apps that needed replacement was Samurize as long unmaintained and not eager to play well with 64 bit Windows version. I was very fine with how Samurize worked for me (my own simple config to sit on top of screen) so I looked for something very similar and Rainmeter widget framework is commonly mentioned alternative.
What it does
Rainmeter itself is an engine that runs in the background and powers up skins (I think widget is more generic term for such). Skins display data retrieved by Rainmeter - from as simple as free drive space to as complex as parsed out of remote web site.
Skins can look like anything and be visually rich (one of skins featured on site):
Or bland and minimalistic (my own work-in-progress skin):
Now that I compare it to my past Samurize config - my ability to put together something visually non-hideous had really improved since.
The way Rainmeter works is very free-form - INI file based text configurations describe skins, which are mostly combinations of measures (data retrieval) and meters (visualization of measure output). Both have vast amounts of parameters and resulting skin can make use of image files (including rich color manipulation) and be interactive (react to data or user input).
App is very light on resources and plays well with most recent Windows versions, including 64 bit builds. You can even easily incorporate Aero transparency features in skins.
While INI format is plain text and one of the easiest to grasp, it is really painful to bend it to describe complex concepts. Rainmeter probably started with functionality for which INI was perfectly sufficient, but more complex things (such as nesting rules and data calculations) are really pushing its limits.
As result it is often frustrating to express your thought in config or get absolutely unhelpful error message (hidden in log) that something that you are doing is not quite right. Pointing out that specific something is often not included.
It is hard to summarize Rainmeter, because how well it will work for you depends on how much you need from it and how dedicated you are to get it. On one end there are people who just want to download and use ready-made skin, on another - willing to study dozens of pages of manual (and maybe dive into Lua script support).
Anyway Rainmeter definitely is powerful and open source software project with native portable install option.