Debug

See Advanced Menu for a summary and video tutorial.
For the Client drop down menu....
Some things are plain to understand while others I have not figured out. I shall proceed in the order of the menu.
== Consoles: ==
The average user won't want to bother with these. However, if you do want to, the texture console can be found at https://wiki.secondlife.com/wiki/Texture_Console . The texture console basically shows the start of a list of texture that are waiting to be loaded, are loading, or have already been loaded, also the memory taken up by them, the level of detail, and other stuff.
The frame console seems to be how much of each frame in the client is taken up by specific processes. (?)
Fast Timers seems to be another way to look at the same thing. (?)
== HUD info: ==
Velocity - How fast and which way you are moving. Your avatar that is.
Camera - The top few pieces are where your viewer is looking at. Alt-Look verifies this. The bottom pieces seem to be about avater location. This information is useful for people making complex HUD's or attachments, or items that follow the camera. 
CameraAtAxis : The rotation of your camera.
CameraCenter : Where the camera is 'sitting'
To understand the difference, imagine this: In RL stand in one spot and look in different directions. Where your head is the camera center, and which way your head is pointing is the CameraAtAxis.
AgentLeftAxis : a unit vector describing which way your avatar is facing physically/on the sever.
AgentRootCenter : the physical center of your avatar
AgentCenter: another measure of the center of your avatar
Something to note: When your avatar is animated, this does not change where your avatar is located on the server.
When in mouselook, your camera moves with animations. When in mouselook, your CameraCenter is dependent upon either AgentRootCenter or AgentCenter. I need to check whether AgentRootCenter changes from AgentCenter when sitting on a vehicle or chair.
Wind - Obviously gives details on the wind. I haven't played enough with this to clarify the details. (?)
FOV - Field of View. Normally 60.0 degrees, changes with control-8 and control-0 (and control-9).
== High Res Snapshot: == 
For exceptional quality pictures! Best used by those professional photographers.
== Quiet Snapshot to Disk: == 
Removes the camera shutter sound and anim
==Compress Snapshots to Disk: == 
I haven't toyed with it. (?)
==Show Mouselook Crosshairs: == 
The display of that little cross in the middle of your screen when in mouselook.
==Debug Permissions: == 
Makes an advanced permission display show up in your editing menu.
==Clear Group Cache: == 
Clears group panel info cached in memory. This can be useful if the central database has been acting up.
== Rendering == 
This is an important one. Here you can turn off those pesky trees that block your vision, remove the display of that grass that takes up your precious graphics CPU cycles, and much much more!
== Types ==
here are ways to turn off (or on) various pieces of rendering that make up what you see!
Simple: The majority of things you see fall under simple rendering.
Alpha: In most other contexts, I find it means transparency. Some reason I don't see it doing anything for me.
Tree: These are the Linden Tree prim types, NOT custom tree creations. Ya know when you go to create an object there is cube, sphere...tree. Yea that is it.
Character: YOU! Yes, the standard avatar (unreliably including attachments) falls into this one. For some odd reason, disabling this also junks the standard camera avatar follow action.
SurfacePatch: This is the texture of the ground surface itself. Disabling this while keeping Ground on makes the ground a solid color.
Sky: Turns of the sky silly, no atmosphere leave a solid black void up above the horizon.
Water: Turns off the display of standard Linden water, NOT prim water. This will make rivers appear as valleys.
Ground: Careful turn off both this and surfacepatch and you got nothing! Just like when turning off sky, but below the horizon, wait, you ain't got one now!
Volume: Anything that takes up space, in short, all objects, not particles, not avatars, all attachments, 
Grass: Yep, cousin to Tree. Removes Grass prim type.
Clouds: Live in a skybox and can't stand the fog in your room? Turn this off! Hello to clear skies!
Particles: Getting spammed by particle effects and want it off? Use this! Take note is also removes the selection beam.
Bump: This only makes a difference if in preferences you enable shiny & bump mapping.
== Features == 
These don't quite fall under rendering types, but they also are worth taking a look at.
UI: If you uncheck this, you won't be able to find your way back to turn it on! UI stands for user interface and includes all menus and buttons. If you want to go completely full screen, you could do this at your own peril.
Selected: Not sure...
Highlighted: When you select an object for editing, that outline you see of the object is this feature.
Dynamic Textures: 
Foot Shadows: Ever look at your feet closely while on the ground? Yes there is a shadow and this is it! The rendering method of this has been discussed somewhere.
Fog: Have you noticed how there is a haze in the distance, a haze which you can set in preferences? This totally turns it off. Be prepared for things to suddenly appear!
Palletized Textures: Not sure...
TestFRInfo: Not sure...
Flexible Objects: Those flexiprims lagging you too much? Turning them off here freezes the prim in its current shape.
Info Displays - Here things can get wacky, perhaps psychedelic if you are into that.
== World ==
Mouse Moves Sun: Lets you drag the sun around to get that perfect snapshot.
Sim Sun Override: (?)
Dump Scripted Camera: Using a scripted camera and it goes beserk? This will unstick you just like Stop All Animations, for cameras.
Fixed Weather: (?)
Dump Region Object Cache: (?)
UI - I'm afriad to disable it. I ain't touching this other than speculation. (?)
XUI - I ain't touching this with a two foot bannana. (?)
Character - Stuff to do with your character, mostly.
Network - Useful tools that have to do with network and waiting for it.
Enable Message Log: (?)
Disable Message Log: (?)
Velocity Interpolate Objects: Remember in old releases when you would lag and start walking through building and into the ground? That is an example of this. Otherwise, the client won't show your new position until it gets confirmation from the server. Personally I prefer this on, so I don't suddenly stop walking.
Ping Interpolate Object Positions: Not sure... (?)
Drop a Packet: Perhaps when a packet gets jammed in the pipeline? (?)
== Recorder ==
For Machinima? (?)
==Show Updates: ==
Creates an expanding shape rise from anything that changes on your client. See https://support.secondlife.com/ics/support/default.asp?deptID=4417&task=knowledge&questionID=4636 for an explanation.
== Compress Image: == 
(?)
== Limit Select Distance: ==
Try selecting an object for editing far in the distance, no I mean farther. You can't right? So disable this and you could in principle select something a sim away if you could see that far.
== Disable Camera Constraints: ==
Do you Alt-Camera extensively? Notice there is a distance at which you are stopped? This it what to check to go farther.
== Joystick Flycam: ==
I don't have a joystick. (?)
== Mouse Smoothing: ==
I haven't noticed much. (?)
== Console Window: ==
Doesn't do what I expected. (?)
== Clothing...: == Appears to bring up a window listing all the clothes you have. Not attachments.
== Debug Settings: ==
Debug Settings
== View Admin Options: ==
Enables fly in no fly zones, TP from profiles without friendship, and seeing where IM's originate