1. FreeBSD：
FreeBSD是FreeBSD项目提供的一个免费的开源操作系统。它是基于两个类似Unix的操作系统Berkeley Software Distribution(BSD)和Research Unix设计的。它与Linux操作系统非常相似。它是较早的著名操作系统之一，但是现在与其他操作系统相比，它的使用率有所降低。 FreeBSD的第一个版本于1993年发布。FreeBSD的最新稳定版本是12.1。
2. OS / 2：
OS / 2是由Microsoft Incorporation和IBM协调开发的一系列操作系统。它的目标系统是个人计算机和服务器。它使用C，C++和汇编语言编写。 OS / 2的第一个版本于1987年发布。它于2006年停产，此后以eComStation的名称进行更新，维护和销售。在2015年，它被命名为ArcaOS。此操作系统中使用的内核是带有模块的Monolithic。
OS / 2和FreeBSD之间的区别：
| OS/2 | FreeBSD |
|---|---|
| It was developed by co-ordination of Microsoft Inc. and IBM. | It was developed and is owned by The FreeBSD Project. |
| It was launched in 1987. | It was launched in 1993. |
| Its target system types are personal computers and servers. | Its target system type is server, workstation, NAS and embedded systems. |
| Computer architectures supported by OS/2 are 16-bit x86 (1.x only) and IA-32. | Computer architectures supported are IA-32, x86-64, ARM, MIPS and PowerPC. |
| Its kernel type is Monolithic with modules. | Kernel type is also Monolithic with modules. |
| Its package management is Feature Install. | Package management is Port collections and packages. |
| Its native APIs are Proprietary, DOS API and Win16. | The native APIs are BSD/POSIX. |
| It has the preferred license of Proprietary. | Preferred license is BSD. |
| File systems supported by OS/2 are HPFS, JFS, FAT, ISO 9660, UDF and NFS. | File systems supported are UFS2, ZFS, ext2, ext3, FAT, ISO 9660, UDF and NFS. |