*Nix4all

Well not exactly "on the fly" you actually need a restart but nonetheless I am not aware of any other OS being able to do this.

HP-UX (Hewlett Packard UniX) is Hewlett-Packard's proprietary implementation of the Unix operating system, based on System V (initially System III). It runs on their PA-RISC range of processors and Intel's Itanium processor, and was also available for later Apollo/Domain systems. Earlier versions also ran on the HP 9000 Series 200, 300, and 400 computer systems based on the Motorola 68000 series of processors, as well as the HP 9000 Series 500 computers based on HP's proprietary FOCUS processor architecture.

Solaris is a Unix-based operating system introduced by Sun Microsystems in 1992 as the successor to SunOS.

Solaris is known for its scalability, especially on SPARC systems, as well for being the origin for many innovative features such as DTrace and ZFS.[1][2] Solaris supports SPARC-based and x86-based workstations and servers from Sun and other vendors, with efforts underway to port to additional platforms.

AIX (Advanced Interactive eXecutive) is the name given to a series of proprietary operating systems sold by IBM for several of its computer system platforms, based on UNIX System V with 4.3BSD-compati

Unix(officially trademarked as UNIX, sometimes also written as Unix with small caps) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. Today's Unix systems are split into various branches, developed over time by AT&T as well as various commercial vendors and non-profit organizations.

As of 2007, the owner of the trademark is The Open Group, an industry standards consortium. Only systems fully compliant with and certified to the Single UNIX Specification are qualified to use the trademark; others are called "Unix system-like" or "Unix-like".

During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (particularly of the BSD variant, originating from the University of California, Berkeley) by commercial startups, the most notable of which is Sun Microsystems. Today, in addition to certified Unix systems, Unix-like operating systems such as Linux and BSD are commonly encountered. Sometimes, "traditional Unix" may be used to describe a Unix or an operating system that has the characteristics of either Version 7 Unix or UNIX System V.