Installation:
Put UIInstaller.exe and ICSharpCode.SharpZipLib.dll in the same folder.

Usage:
Put a UI zip named EQ2Interface.zip in the folder with UIInstaller.exe and double-click UIInstaler.exe.
or
Drag-and-drop a UI zip onto UIInstaller.exe.
or
Double-click an EQI file. (Associate with EQI files must be checked; see UIInstaller-Main.png screenshot)

Click the Find button to have UIInstaller locate your EverQuest II UI folder automatically. (Optional; see UIInstaller-SelectLocation.png screenshot)
Click the ... button to create or specify the folder within UI to install the UI to.
Click the Install button.

Notes:

UIInstaller will not allow you to install UIs unless the target folder is a direct child of UI. (E.g. C:\EverQuest II\UI\Mods is good, C:\EverQuest II and C:\EverQuest II\UI\Mods\images are not)

UI zips must retain their folder structure and may not contain extra parent folders. (See UIInstaller-ZIP.png screenshot)

This download includes a sample zip containing ger's Loc Icons.