The most famous current language that has a dynamic syntax is Perl6. The most famous example in general is probably Smalltalk-72.
You might also want to look into OMeta and the rest of the STEPS project from Alan Kay's Viewpoints Research Institute, as well as Val Schorre's original META II.
Also interesting: the π programming language.
XL (Extensible Language) also allows for powerful manipulation of the syntax itself.
