BLASM

BLASM is a free Assembler which is fully customizable - you can add and modify everything like processor registers, executable headers, segments, output formats, instruction lists and so on. Support for Intel- and AMD-processors is included in the package. The assembler is command-based and can run on multiple platforms - binaries for Windows and Linux is included in the package, but you can also compile the source code and run BLASM on other platforms. BLASM is relased as open source under the GPL (General Public License).