Shmup-base-engine is a project mainly written in C++ and C, it's free.
A shoot 'em up engine created in SFML with support for Windows and GNU/Linux.
The Shmup Base Engine or SBE for short is a programming school project by Felix, Victor and Jonathan, with the goal of creating a fully featured shoot 'em up engine in C++ with the multimedia library SFML. The source code is licensed under the GPLv3.
More info will follow. UNLESS EVERYONE DIES IN A HORRIBLE ACCIDENT OF SOME UNDETERMINABLE NATURE