The ECMA Standard
The second edition of the ECMA Eiffel Standard came out in June, 2006. Three compiler projects are supporting this standard. EiffelSoftware's EiffelStudio, the Gobo Eiffel Compiler and "The Eiffel Compiler" tecomp.
Available under a dual license for Open Source and commercial projects, EiffelStudio is the granddaddy of Eiffel compilers. Produced by EiffelSoftware (formerly ISE), EiffelStudio emphasizes backward compatibility combined with incremental language enhancements. It is probably the most widely used Eiffel compiler.
2013 a new Compiler based on SmartEiffel is in devolepment. It is the new GNU Eiffel Compiler.
The Gobo Eiffel Compiler
An offshoot of the Gobo project, the Gobo Eiffel Compiler, or GEC, is currently under development and as of this writing does not support a full implementation of the language. It does contain a complete parser that supports several versions of the language, including compatibility with EiffelStudio and large portions of the ECMA standard.
These two compilers actually have a close relationship. EiffelSoftware has made extensive use of the Gobo libraries to implement their own compiler. Eric Bezault, the founder of the Gobo project, is working with EiffelSoftware on an open source ELKS library to be shared by both compilers.
"The Eiffel Compiler" tecomp
The development of "The Eiffel Compiler" tecomp started in 2008. It is a compiler and an interpreter at the same time. It compiles the Eiffel source code into its internal representation and either stores it for later execution or executes it in its virtual machine. Optimized native code can be generated via compilation to C.
ELKS was originally formed to develop standards for Eiffel libraries usable by all Eiffel compilers. It laid out some very solid groundwork for libraries earlier this decade.
(TBD: Provide relevant links for more information on all of the standardization efforts and products.)