Site: http://itdelatrisu.github.io/opsu/
Releases: https://github.com/itdelatrisu/opsu/releases/
About
opsu! is an unofficial osu! client written in Java using Slick2D and LWJGL, wrappers around the OpenGL and OpenAL libraries. The only requirement to run opsu! is a Java Runtime Environment (JRE), so it's cross-platform compatible (tested on Windows, OS X, and Ubuntu). fluddokt has also ported the project to LibGDX, so opsu! will also run on Android (and theoretically iOS, but that's untested). More details can be found on the opsu! web page or the GitHub readme page.
Running
If you've installed osu! in the default program folder, opsu! will run from anywhere: just execute opsu-{version}.jar and you're set. Otherwise, you can either set the paths manually in the configuration file (.opsu.cfg, generated when the program starts) or run opsu! from the working directory. There's a built-in beatmap downloader, and any OSZ archives placed in the SongPacks directory will be automatically unpacked.
Game options can be accessed by clicking the "Other Options" button in the song menu. You'll likely need to change the "Music Offset" value if hit objects are out of sync (this might involve a bit of playing around -- it's a known issue, sorry!).
Project Goals & Contributing
Update: This project has reached a relatively stable point, thanks to all the user contributions I've received -- please keep sending them in! If you have any interest in the project, feel free to contact me by opening an Issue on GitHub, and I'd love to hear your thoughts.
Credits
The majority of images included in opsu! are taken from user-released skins, and their names are listed in the credits file in the repository. I also referenced the Wojtkosu and osu! web projects when writing portions of the code. fluddokt also contributed significantly to the project. Lastly, I want to thank peppy for allowing me to share this here.
Screenshots
See this page.
License
This software is licensed under GNU GPL version 3. You are free to change and redistribute it, but only under the terms of the license. The full text can be found here.