Allegro is very old #gamedev library, written in C, but still in active development.
Allegro is a cross-platform library mainly aimed at video game and multimedia programming. It handles common, low-level tasks such as creating windows, accepting user input, loading data, drawing images, playing sounds, etc. and generally abstracting away the underlying platform. However, Allegro is not a game engine: you are free to design and structure your program as you like.
Allegro 5 has the following additional features:
- Supported on Windows, Linux, Mac OSX, iPhone and Android
- User-friendly, intuitive C API usable from C++ and many other languages
- Hardware accelerated bitmap and graphical primitive drawing support (via OpenGL or Direct3D)
- Audio recording support
- Font loading and drawing
- Video playback
- Abstractions over shaders and low-level polygon drawing
- And more: https://liballeg.org/a5docs/trunk/getting_started.html