These are my favourite Google gadgets for Google Desktop 4. Some are new, others were available in Google Desktop 3, but most are extremely useful and have a big advantage over standalone applications: you can see their display all the time.
System Monitor - Shows CPU usage, memory usage, disk activity, network activity.
Desktop Netstat - Monitor your computer's network connections in realtime. You can view all TCP connections with local address, remote address and owning process for each connection.
Desktop News - View the latest news from Google News. If you enable Advanced Features in Settings, the news will be tailored to your taste. If you click on many tech news, you'll see more news about science and technology.
Digg Top Stories - You'll never miss an interesting news from Digg from now on.
Wireless Signal Meter - A gadget created by Google that shows wireless signal strength.
LaceLevel2 - Monitors fragmentation level of your local drives. This way you'll know when you need to defragment your hard-drive.
AdSense Status - View how much have you earned from your Google AdSense account.
Google Calendar Gadget - You can manage events and see what's going on this month without going to Google Calendar site. The definite Google Calendar plugin.
gdShredder - Securely shred files/folders using drag and drop, the recycle bin, and free disk space so that the data can never be recovered.
What's Hot - Tracks popular news and blog posts from Technorati, BlogPulse and other sites.
To add these gadgets, download Google Desktop 4, click on "Add +" button from the top and search for the name of the gadget. Most gadgets can be added instantly.