XGuetzli

Google has recently released Guetzly, an open-source encoder that enables you to create small, high-quality JPEG images using a lightweight command-line utility. Naturally, though, not everyone likes using the command console, which is where this application comes into play.
XGuetzli is a relatively simple utility that serves as a graphical user interface for Guetzly, ideal for those who find the original encoder to be rather unintuitive. It can also optimize files using Leanify, but it lacks a couple of important features.

A few words about Guetzli, the software the powers XGuetzli

Google has developed and released an open-source algorithm that is capable of creating JPEG images up to 35% smaller than those created with other well-known encoders. The resulting images also feature less artifacts, but the encoding process is very time-consuming.
The utility is only available in the form of a command-line tool, which can be used to encode both PNG and JPG files. It is not particularly tricky to use, but many people understandably shy away from command-line applications.

Encode JPEG files using an intuitive GUI

XGuetzli is remarkably easy to use, and it allows you to process multiple files at once, but it would have been great if items could be added to the conversion queue using drag and drop.
Also worth noting is that, unlike Guetzly, this application can only process JPEG files at present. The output quality can still be customized, but you cannot set a memory limit.

Optimize files after they are encoded using Leanify

Although this step is entirely optional, it is possible to further reduce the file size of JPEG images by running them through Leanify, a utility that removes unnecessary data from files, such as metadata and debug information, without affecting picture quality.
However, the application needs to be downloaded separately, and XGuetzli will only be able to use it if the executable file is placed in the same directory.
All in all, this is an interesting alternative to Guetzly for users who prefer to have an intuitive GUI at their disposal when encoding JPEG images. Granted, the user interface is rather simplistic, but it is a lot more user-friendly than the standard console application.