TenDRA is a free, OpenSource C/C++ compiler. It was originally developed by the UK's DefenceEvaluationAndResearchAgency?, and is still mostly under Crown copyright, but has been released as an independent OpenSource project under the BSD license.
TenDRA was developed as a tool for working with the ArchitectureNeutralDistributionFormat?, but is a useful compiler in its own right. It can handle C and C++, with STL support in the works. It can emit native code for x86, Alpha, SPARC, HP-PA, MIPS, Power (but not PowerPC) and 68k instruction sets. The core of the compiler is very clean and sophisticated, but as a working tool, it's not as mature as the GnuCee compiler or a commercial compiler.
TenDRA is somewhat popular in the BSD world, where some people are unhappy about depending on the GNU project's C compiler.
None of the above links work (20120824) This WikiPedia page indicates that the projects faded out about 2007: http://en.wikipedia.org/wiki/TenDRA
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