Knuth has declared that he will do no further development of TeX; he will continue to fix any bugs that are reported to him (though bugs are rare). This decision was made soon after TeX version 3.0 was released; at each bug-fix release the version number acquires one more digit, so that it tends to the limit π (at the time of writing, Knuth's latest release is version 3.1415926). Knuth wants TeX to be frozen at version π when he dies; thereafter, no further changes may be made to Knuth's source. (A similar rule is applied to MetaFont; its version number tends to the limit _e_, and currently stands at 2.718281.)
There are projects to build substantial new macro packages based on TeX: the most notable is The LaTeX3 project. There are also various projects to build a successor to TeX-the-binary. The success of these projects has varied: today, pdfTeX is the standard engine for most users, whilst XeTeX and LuaTeX have both delivered fully Unicode-compliant systems with contrasting approaches to extensibility.