With GnoPS (Gnomish Positioning System!) you can create pings for finding exact co-ordinates to within 0.01 x/y's.

Make a greeting for someone warping to your cornerstone or entering your house! Have a message in your favourite meeting spot for others to find!
Or with the "secret locations" option, create a starting point for a treasure hunt, or story (for those RP'ers out there). Locations following the first can't be found until the previous step has been triggered.

Simple repeatable locations allow a greeting message, whilst secret locations allow a greeting message followed by a second option for more flavour.

Currently it can be controlled via a list of typed commands, starting with /gnops showing the available options. Standard system "/gnops <command>" to carry out the action.

list > Shows current list of locations (non-secret)
log > Shows a log of all secret locations with their "story" and "content" lines.
share > Shares locations with targetted player (/gnops share "<location name in full>")
sharesecret > Shares secret locations with targetted player (/gnops sharesecret "<location name in full>")
resetchain > Resets a secret chain (for example "/gnops resetchain #01" would reset secret chain #01 as shown in the log)

To add/remove locations, normal and secret, there is a basic gui accessed with /gnopsgui

Adding a normal location is as simple as standing where you want it, selecting "Add New" on the Command drop down, giving it a name, and hitting "Okay".
Adding a secret location is similar. Checking the "Secret?" box opens up a few extra boxes.

#Chain > enter in numbers to allow secret chains to work with each other, eg. [01] - [01], in the second step you would enter [01] - [02] and tick the "Require Prev"
Story > This is the first line that shows upon discovery of the secret location
Content > This is the text in the second window that pops up after clicking the "Story" okay button.

To remove locations, select "Remove" on the drop down and type the location name exactly as is. To check you can use "/gnops log" or the Saved Locations on the left of the gui.

Notes:
Normal locations will ping every time you re-enter the location, whilst secret locations will only ping once unless resetchain is done.
Currently it's just a default bell sound. This may become customisable down the line.
There may be issues with overlapping chain numbers, however I haven't managed to trigger these myself. Chains link not just via number, but via title of previous step, so keep those unique!

To do:
Entering co-ords currently are restricted to whole numbers, e.g. 56, 65. I'll fix this for decimals. You can still stand where you want the location and it'll save with precision (while leaving the x/y blank).