All of the systems I've talked about so far are well established. However, there are newer systems available that are built on the foundation of older systems, namely TADS 3 (often called T3) and Glulx.
TADS 3 is the newest version of TADS, and is very much under development. TADS 3 uses a different virtual machine than TADS does. T3 game files end in .t3x, to distinguish them from older .gam files. T3 interpreters are available from the /if-archive/programming/tads2/executables directory of the IF Archive.
Glulx is Andrew Plotkin's new virtual machine which is designed to work with Inform and to sidestep some of the limitations of the z-machine. Glulx game files end in .ulx. Its interpreter is called Glulxe, so that there is no possible way that you would confuse it with the virtual machine itself. You can get a copy of Glulxe for your machine from the /if-archive/programming/glulx/interpreters/glulxe directory of the IF Archive.