Drizzle is a powerful Uru tools with many features covering very different aspects of the game. Most notably, it features the Uru Age Manager and a tool to convert ages from various Cyan games to Uru.
Drizzle was originally written by Dustin. Now is unmaintained and therefore will not receive new features or bugfixes anymore. However, the main features are quite stable, and in case of problems, just ask in the Drizzle subforum. Maybe some other user can help you.
Drizzle requires Java to run, so you first need to get that from the Java website unless you already have it installed.
Now you can head to http://sourceforge.net/projects/drizzle/ to download the latest version of Drizzle. "Drizzle??.jar" is the actual program, it should usually be launched on double-click. If instead some archive application starts and shows the content of that file, the default launcher is configured incorrectly. Right-click on the Drizzle file, choose "Open with", and then select Java to properly run the application.
Fan age download and installation
This Drizzle functionality is covered by the Uru Age Manager article
Cyan age conversion
Drizzle is able to convert the ages of Myst Online: Uru Live, Myst V, Crowthistle, Hex Isle and MagiQuest to the format of Uru: Complete Chronicles so that they can be played offline, with the Uru avatar and all the powers of the Offline KI. Since Alcugs Shards also use that version of the game, you can even play ages online that were never available for multiplayer before.
The conversion process is the same for all supported games: Choose the "Conversion (Simplicity)" tab of Drizzle. First you need to select the folder of your Uru installation ("Pots folder"), which will be the target for the conversion. Then you can select the folder you installed the source game to, and finally hit the "Start..." button next to that selection. The conversion can take up to ten minutes on slow systems, depending on which game you selected. To reach these ages within Uru, you will also need to install the Offline KI.
There also is a "Start..." button next to the "Pots folder" selection: Hitting it will apply some patches to your Uru installation that make the Offline KI even better. For example, this allows for fan ages to get their own custom linking images and book covers in Relto and Nexus.
3ds Max age conversion
Drizzle is also able to convert ages you created with 3ds Max to the format of the old Uru versions, see the exporting tutorial.