TransMouse

TransMouse is a simple application that you can use to personalize your operating system by adjusting the transparency level of active windows. It has a handful of options. However, there are a couple of observations that must be made related to how it works.

Portable tool with systray integration

It's not wrapped in a setup package. Instead, you can copy the downloaded files anywhere on the local disk or a removable one, in case you want to carry the app with you and directly run it on any PC without previous installers.
It doesn't modify system registry settings, need DLLs to run, or create files on the disk without asking for your approval. However, it auto-updates its .cfg configuration file with your settings on exit.

Easily adjust the transparency level

Once started, it creates an icon in the taskbar notifications area and facilitates quick access to its options. It's possible to move a slider to set the opacity or enter a numerical value instead, ranging from 0 (completely transparent) to 255 (opaque).
Furthermore, you can integrate TransMouse into the system startup sequence to automatically run every time you boot the computer, as well as hide it to the systray with one click.

How it works

Just keep in mind that the utility has to run to keep the window transparent mode enabled. Windows that have been opened before running the application are not affected by it. The remaining ones become transparent, are automatically raised above all the other windows, and can be temporarily restored to opaque mode by moving the mouse cursor over their title bar.

Conclusion

To wrap it up, TransMouse provides you with a simple solution for making opened windows transparent. It's free and has intuitive options for all user levels. CPU and RAM usage was minimal in our tests.