A client-sided tool that allows you to create custom labels and then give those labels to players via command or gui box. You could view your labels by simply pulling up the player list. There would be a new column there called "Label" that would show the label each player has been given. It would be blank if the player hasn't been given a label. You would be able to sort the player list with this button as well, just like you can with names or IDs.
To access the gui, you would type "/labels" into the chat. There it would show you all the labels you have made in a scroll-able window. You could click on a label and then click "View" which would take you to another box that shows you every player that has been given that label. In this box you could also add names/IDs to the list or remove them, as well as edit the label itself. Labels are given to the player's ID, so if they change their name they would still have the same label.
Back to the main gui. There would be a button to create a new label and a button to delete a selected label. You would also be able to create labels through the chat by typing "/createlabel (labelname)"
Once you have created a label, you would then be able to add players to that list through the chat. Let's say you've created the label "Regular" (a player that is a regular at your server). You would then be able to type in the chat "/label Blok regular" which would add the player to that list. Once you've done that, in the chat it would tell you "Blok has been added to Regular" so you know you've done it correctly. You could also remove players from a label by typing "/clearlabel Blok" which would then say in the chat "Blok has been removed from Regular." You would only be able to use these commands if you have created that specific label either through the gui or through the chat command.
The point of this would be to allow you to sort players based on whatever you like. My example would allow you to easily see what players are regulars at your server so you know that they understand the rules or that they are experienced at your server. This could be used for a number of things depending on your needs.
What do you guys think? Please suggest new ideas. If I like them I will add them to the OP.
