On my Windows machine I press Winkey + C and a calculator pops up. I find this very useful - is there any way to achieve this in Ubuntu?
Assuming you're using 11.10:
You can set a key for the calculator there by clicking the shortcut key in the row listing the shortcuts, and then pressing the desired shortcut keys on your keyboard. You can also set keys for any non-standard apps you like in the 'Custom Shortcuts' category. The only problem is finding a key that hasn't been used for something else.