Using adaptive grammars

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.

The Katahdin programming language offers syntax and semantic changes at runtime. It's an unsupported thesis language so don't expect to use it in production. Still, Chris Seaton's thesis may be a good resource for implementing similar ideas in your own language.

Seaton is cited in this paper describing a macro system for the Fortress programming language. The authors' goal is to make syntactic extension indistinguishable from core syntax.