I use Google Calendar at work and I have access to lots of my coworkers' calendars which makes the view pretty messy, so I mostly keep them hidden. Is there a way (maybe with some plugin or extension) to show and hide all these calendars with one click?
I realize that you are specifically asking for one click, still I'd like to offer a (mostly) three click solution as likely helpful as well (for anyone not aware of it already at least ;)
Obviously, depending on your calendar distribution between the upper and lower section, you might need to click all/none another time in the other section as well eventually.
In case this really doesn't serve your needs, it should indeed be possible to extend Google calendar via the Calendar APIs and Tools (the calendar class has a hidden property) and/or a Calendar Sidebar Gadget - I'm not aware of an existing solution offering the desired functionality, so you'll need to roll your own in case.
There is no one-click solution for this, and no keyboard shortcut that I know of, but there is a two-click solution.
Under "My Calendars", find your calendar (It's at the top of the list), click on the down arrow and select "Display only this calendar".
That's two clicks ;)
Sadly, there is no way to restore the calendars to the previous view (before you selected "Display only this calendar").
Edit: Well I re-read your question. I guess my answer is obvious, but it does answer half your question ;)