Opera Mini has some special commands that you enter in the address bar like a URL but without the "http://". Many of them return information that's probably only of interest to Opera's developers but a couple are actually very useful.
My favorite is config: which opens Opera Mini's Power User Settings page where you can tweak many browser settings like the timeout and whether the browser should make phone numbers clickable. See my post Opera Mini's Hidden Power User Menu for more information.
Web developers will appreciate server:source which displays the source code of the current page.
Here's a full list of all the hidden Opera Minicommands that I've discovered. I've tested these in Opera Mini 4.3 and 6 and I believe that (except as noted) they should work in all Opera Mini versions since 4.0.
config: - Open the the Power User Settings menu
server:source - Display the current page's source code
opera:cache - displays the size and URL of cached pages and images
about: - displays information about the Opera Mini server
debug: - Displays a page listing many Opera mini server and client properties. The "Host:" field tells you which Opera Mini server you are currently connected to.
The first letter of the host is supposed to indicate which country the server is in. I believe that "r" hosts like "r02-12" are in the US. as I'm in the US and I'm usually connected to "r" hosts. I've also seen or heard of "t", "z" and "v" hosts. Anyone seen others or know where any of them are?
The following commands duplicate Opera Mini menu functions.
feed:list - Go to the Feeds page
opera:about - Displays copyright and license information for third party software used in the browser
server:reset - clears all cookies
server:refresh - refreshes the current page
o:A - goes to the start page (Opera Mini 4.n only)
o:Z - goes to your bookmarks (Opera Mini 4.n only)
I suspect that there are more of these hidden commands. If you've discovered others that are currently working, please lets all know in a comment.