Syllable is a free and open source operating system for Pentium and compatible processors. Its purpose is to create an easy-to-use desktop operating system for the home and small office user. It was forked from the stagnant AtheOS in July 2002.
It has a native web browser (ABrowse which is WebKit-based), email client (Whisper), media player, IDE, and many more applications.
Features according to the official website include:
Native 64-bit journaled file system, the AtheOS File System (usually called AFS, which is not the same as the Andrew File System)
C++ oriented API
Object-oriented graphical desktop environment on a native GUI architecture
Mostly POSIX compliant
Software ports, including Emacs, Vim, Perl, Python, Apache and others.
GNU toolchain (GCC, Glibc, Binutils, Make)
Pre-emptive multitasking with multithreading
Symmetric multiprocessing (multiple processor) support
Device drivers for most common hardware (video, sound, network chips)
File system drivers for FAT (read/write), NTFS (read) and ext2 (read)
REBOL as system scripting language