This field is interesting and perhaps a good solution for many companies areound the world . I felt forced to add som news aswell as a compliment to the good post by RA.

"SuSE Linux Enterprise Server 7 For S/390" Ready for IBM eServer zSeries
Oakland, California, 11/12/2001.

SuSE offers ideal solution for consolidation of mission-critical e-business applications with kernel 2.4, HiperSockets support, Logical Volume Manager and Journaling Filesystem

Today, SuSE Linux, the international Open Source technology leader and solutions provider, announced the release of SuSE Linux Enterprise Server 7 for S/390, the latest version of its server operating system for deployment in enterprise mainframes. Based on kernel 2.4, this SuSE server version now supports S/390 servers as well as IBM eServer zSeries z900.

The full press release from SuSE can be read here.

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Red Hat Linux 7.2 for S/390.

Red Hat Linux 7.2 for IBM®'s S/390 mainframe architecture offers the flexibility and value of Linux combined with availability and reliability of S/390. The result is an industrial, enterprise strength Linux environment that lets you cost effectively manage your mission-critical IT workloads.

The full press release from Red Hat can be read here.