This is another one for those of you who are relatively new to Second Life!
You may not know it, but there are two "hidden" menus in your viewer -- the Advanced and the Develop menus. (Some viewers don't have the Develop menu, but never fear...all the settings are located in Advanced, or in other menus instead.)
These two menus are hidden for a couple of reasons. One, to keep the user interface relatively simple and uncluttered for newcomers, and two, to keep the curious but clueless from doing something to Mess Things Up.
But these two menus contain a lot of things that are very useful. I keep them on my top menu bar all the time. To see the Advanced menu, hit CTRL+ALT+D. Then, to see the Develop menu, hit CTRL+ALT+Q, or go to Advanced/Show Develop.
THE ADVANCED MENU
Disable Camera Constraints - Check this to let you move your camera much farther away from you, and/or into and through objects.
Hi-res Snapshot - lets you take snapshots with more pixels
Quiet Snapshots - disables the shutter sound and the "take a picture" animation
Rendering Types - Use this to temporarily disable things you don't want to see. Useful for turning off particles if a poofer is being annoying, or for turning off water or object visibility to see parcel boundary lines.
Release Keys - this can be useful in certain situations where you seem to be permanently stuck in an animation. But try Avatar/Avatar Health/Stop Animations, or Stop Animations and Revoke Permissions, first.
Fly Override - This will let you fly on land where the owner has disabled flight. Be aware that some land owners get peeved when you ignore their land settings, and this could get you banned from that land.
Restrained Love API - This enables features that allow another person to exercise a lot of control over your avatar and your Second Life experience. It's most commonly used by people in the BDSM community to enhance that type of roleplay. Enabling or disabling this also requires a re-log. Not recommended unless you know what you are doing.
Debug Settings - this opens a window with a TON of obscure viewer settings. Don't mess with them unless you know what you are doing...but if you do, and you mess up, you can always change back to the default settings. There is a Second Life Wiki page that lists all the settings, and gives a little bit more information about what each one does. http://wiki.secondlife.com/wiki/Debug_Settings
THE DEVELOP MENU
Show Info - this has a lot of sub-choices. The most useful is "show color under cursor." When checked, three small numbers will appear in the lower right corner of your viewer, showing the RGB values of the pixel under your mouse cursor. Useful for doing color matching.
Rendering - another one with a lot of sub-choices. The most useful is Wireframe. Toggle this on, then off (the shortcut is CTRL+SHIFT+R) to force objects in view to appear.
UI - make sure the "Double Click Teleport" item is checked. This allows you to teleport to your mouse cursor by double clicking. Note: This may fail if the land owner has limited teleports to the arrival point.
Avatar - the most useful sub-items are Character Test, to put on a default basic avatar if your current appearance refuses to render, and Show Look At, to see where other avatars' point of view is. But do recall that simply because someone's viewpoint crosshairs seem to be on you, they may not actually be perving you. I leave this turned off except in special cases, to keep my blood pressure down. Playing with the Animation Speed settings can be amusing, too.
Request and Leave Admin Status won't do anything, unless you happen to be a Linden. If you are, it enables "God Mode" and makes you omnipotent.
Enjoy your new mastery over your Second Life experience!