Have you played with Google's Desktop tool? This is basically a strip on the side of your screen that lets you house small applications, called Gadgets. The tool is available for Windows, Linux and Mac so no matter your vice there is a flavour for you.
In addition, Google has a whole set of API's allowing you to query details about the system in which you are running. For example, you can determine if they are running a wireless connection and then display various realtime settings regarding their connection.
Creating a gadget is a relatively painless operation.
First step is to download the Google Desktop application itself and get it all installed. After that, download their SDK, which is a collection of sample gadgets, reference documentation and a very basic IDE.
A word of warning here, use the IDE only as a jumping off point. It is very buggy and doesn't let you do a lot of things. Instead use it more like tutor/explorer.
The IDE lets you create a Gadget project and use a drag'n'drop interface for laying out your gadgets UI. However after that, the event plumbing is buggy and you can waste a lot of time trying to figure out why events aren't being fired or picked up. The bug lies in the editing of the main.xml file; it neglects to put in the event handlers when you define the project. The documentation is a little light on this front, so keep an eye out for that gotcha. Ironically, the IDE doesn't actually let you hand edit the XML files. For that you have to use an external editor, and then the IDE will pick it up.
The main entry point for every gadget is the main.xml file. This defines the UI and lets you define what handlers you want to listen. If you really must, you can think of this like a small HTML snippet, but instead of using HTML, you are only allowed to use the tags that Google defines.
<view height="100" width="250" onopen="view_onOpen()" onoptionchanged="onOptionChanged();"> /* your gadget layout */ </view>
Google Desktop manages the state for you, allowing you to save users settings by utilising a popup Options box. This is a simple matter of marshalling data in and out of a hash map; you place data in there with a key and pull it out with the same key. Google will store the data on the user's computer and will make it available when the desktop application restarts.
After that, you are then left to your imagination with regards to what your gadget is actually going to do. They do recommend that you don't get too excited and try and hog the users CPU or network. Moderation people!
Packaging up your gadget for distribution is very easy. It is basically a zip file, that has had the extension renamed to .gg. The quickest way is to just hit the Package option inside the IDE. The IDE will also create all the basic necessary files for you. There are a number of XML files that you probably want to fill in that has details about your gadget that will be presented to the user at various stages (initial install, when they click Install, when its listed on Google, etc).
That is it. Simply email the gadget or upload it to a server and as soon as someone downloads it, the Google Desktop will grab it and ask if you want to install it. Very painless.
Overall, the Google Desktop application doesn't consume too many resources or at least nothing that I noticed. However, there are lots of buggy gadgets out there, even some of the official Google one's caused some grief for me. It's definitely a lot lighter than Yahoo's Widget alternative, which I had to remove only after a day after it wouldn't let the CPU fall below 10%.
I built a gadget for use with your Blog-City blog. This one monitors the hit rate of your blog, showing you a counter on your Google Desktop to the total hits.