My Mystitool HUD sits at the bottom right of the screen, visible as two translucent circles. It's always attached, and I feel naked without it.
That W button here? That's an add-on of my own creation. I made it so I could turn my walk on whenever I switch off my AO. I do that sometimes so sits and other animations will play correctly. When I forget to switch it back on the Walk button stops me from having that lovely newbie waddle.
Just above the HUD is the Mystitool's display of nearby avatars. In this case Exuberance Lafleur is five meters away. That tells me she's within easy chat range and will hear me when I talk-- and even if I whisper.
Now I've walked a ways up the hill. Xubi is visible, but I know she's beyond the twenty meter limit for chat and won't hear me unless I shout. I also know she is sitting and away. Similarly, I would know if she were flying, in the air but not flying (falling, most likely), walking, typing, busy, crouching, in always run mode, or on Auto-Pilot, which I suppose is what happens when have your viewer set to make you walk when you click on the land.
I can also tell from there are only two avatars on Whimsy-- Exuberance and myself.
The Mystitool notifies me in chat when an avatar comes into chat range. I can't tell you how many times that has stopped people from sneaking up on me!
Firestorm's built-in radar will do many of the same things as the Mystitool, and more besides. I use it often to locate and teleport to Sweetie-- but I still rely primarily on the Mystitool. I keep Firestorm's radar chat functions turned off so they won't duplicate the Mystitool's notification.
The Mystitool has a remote avatar scan function which will let its owner know the names of avatars on other sims. This requires the Mystitool owner to either own that region or set the remote to the land's group.
This means I can check my sims no matter wherever I may be on the grid. "Nope, Sweetie's not at home. She must have gone in search of that mesh fern she was talking about. I'll need to IM and ask her for a teleport assist."
A handy bug scanner will show all objects with active scripts within 30 meters (the range can be set to any distance). This is especially handy when I'm working around Whimsy. "That buzzing bee noise is driving me crazy. Which of these many plants did I put that sound script in?"
I'm always finding out new things about my Mystitool-- sometimes because I've not read to the bottom of the notecard, and sometimes because new features have been introductd. Just now, in writing this, I discovered I can turn on a sound that will play whenver an avatar comes within chat distance. How handy that will be when I have the screen minimized!
If I were not so accustomed to my Mystitool I would be happy to rely on Firestorm's notifications-- but I find it reassuring to look at the lower right hand corner of my screen and know who is nearby.
Here's the Mystitool's Privacy notecard in its entirety:
MystiTool Multipurpose HUD
by Mystical Cookie, since 2006 April
This notecard documents the Privacy features of the HUD attachment.
All of the features described in this notecard are accessible via the HUD menus.
Privacy Features Summary:
Avatar Scanner - /1 scan
Sim-wide AV Scanner - /1 scan sim
Remote Sim-wide AV Scanner - /1 scan remote
Bug Scanner - /1 bugs
Bug Radar - /1 bug radar
AV Chat Range Notify - /1 chat
Repeatedly scans for avatars within the current region. Names will be listed above the MystiTool HUD buttons, up to 21 AVs. The list refresh rate will vary with sim lag.
In very rare circumstances, avatar names may be incomplete in the list of names. If you experience this, please send a screenshot to Mystical Cookie!
At the top of the hovertext, you will see "6 AVs sim-wide ". The  number indicates the total number of avatars in the current sim.
/1 scan on
Enable scanning, unlimited range.
/1 scan [range]
Set scanning range to [range] and begin scanning for nearby AVs.
/1 scan 0
/1 scan resident
Show or hide the "Resident" surname when using Legacy Names.
/1 scan dispname
Set AV scanner and chat range notify to display Legacy Names or Display Names.
/1 scan hogs
Display an exclamation mark (!) next to makes with high sim resource usage.
High resources currently = 100 scripts or more and 3,000,000 bytes of memory or more.
/1 scan stats
Display sim stats (time dilation & fps) under list of names.
/1 scan color
Set the AV Scanner hovertext color.
In addition to scanning for avatars, MystiTool will also list what they are doing after their name.
S = Sitting
F = Flying
H = In the air, but not flying. Falling?
M = In MouseLook
W = Walking
T = Typing
A = Away
B = Busy
R = Always Run
C = Crouching
P = Auto-pilot
Sim-wide AV Scanner
Shows the names of all AVs in a sim.
Full scans may take a few seconds or a few minutes, depending on the number of avatars in the sim and sim lag.
/1 scan sim
Scan an entire sim for all AV names.
Remote Sim AV Scanner
Shows the names of all AVs in a sim, remotely.
You must place the remote scanner object in the sim you wish to scan remotely. Click "New Remote" in the "Scan Remote" menu to rez a new remote scanner object. It will register with your MystiTool upon rez. You may move the object to where you want it to be.
Due to delays in email communication between the remote scanner object and the hud, it could take up to about two minutes to receive your remote sim scan report.
If you accidentally unlist a location or reset your MystiTool, you will need to visit the remote scanner locations again. Simply click them to add back to MystiTool's location list.
To locate a remote scanner object in the sim, you may shout "/1 id" or say "/1 regsay 1 id" while wearing your MystiTool.
PLEASE NOTE: The remote scanner object will only function on land you own directly, or if the object is set to the land's group.
/1 scan remote
Show menu with a list of locations where you have placed remote scanner objects.
This will show all "active scripted" objects within range. The default range is 30 meters, which should include the chat range of yourself and most nearby friends.
Occasionally, you may see *Unknown* as the owner of an object in the Bug Scanner. This happens when the sim returns a null owner key to the script. So far, this appears to happen only with group owned objects. This is a Linden limitation.
Not all "active scripted" objects contain open chat listeners. There is no way to determine if an object is listening to your chat, but if you find "active scripted" objects on your own land not owned by you, you should be suspicious. :)
Due to sensor limitations, only the nearest 16 objects will be shown.
Scan for nearby scripted objects which may be listening to public chat.
/1 bugs all
Same scan, but includes your own scripted objects.
/1 bugs range [#]
Set the range of the scan. Default is 30 meters.
This will rez an object to repeatedly scan the surrounding area for "active scripted" objects. This uses the same scanning method described above.
Click the Bug Radar object for a menu with more options.
Please do not leave this rezzed when not needed. Sensors cause lag. :)
This object may also be attached to your HUD. It will automatically resize itself.
/1 bug radar
Rez a repeating scanner.
AV Chat Range Notify
Notifies you when AVs enter within chat range.
/1 chat on
/1 chat off
/1 chat toggle
Enable or disable chat range notifications.
/1 chat sound
Enable or disable audible chat notifications.
Three chat notification sounds are provided as easy to install patches in the included plugin package.