How can I add or remove a Gadget?

To add more gadgets on your desktop,right click on a blank area of the sidebar and click Add Gadgets...

Now from the gadget gallery you can choose:-

1]Calculator
2]Clock
3]CPU meter
4]Currency Convertersions
5]Feed viewer
6]Feed Watcher
7]Number Puzzle
8]Picture Puzzle
9]Recycle bin
10]Slide Show
11]Sticky Notes
12]Stocks,
13]Timer and
14]Weather.

Now right click on any selected item and select Add or Uninstall according to your need.

You can get more gadgets online.There is also an option to search Gadgets.

Further Reference:-
Getting Started With Gadgets
http://microsoftgadgets.com/