Many of my favorite speakers use extremely old technologies, such as field coil drivers (dynamic drivers that use electromagnets in place of permanent magnets). A lot of newer approaches to speaker design/manufacture sound pretty bad to me (e.g., ceramic coned drivers).
The major recent developments are really outside of the actual speaker/driver field, such as using digital signal processing integrated with the design of the speaker.
Outside of that, the big technological changes that I thought deliver decent sound are all getting pretty long in the tooth. Bending mode drivers (instead of the voice coil moving a cone that is suppose to be rigid and moving in one piece, the voice coil bends the cone and propagates a wave in the cone material itself), such as the Walsh driver and the much improved MBL variant, on such drivers can deliver pretty good sound. I've heard recent designs using Heil drivers that are promising too (Heil drivers are essentially planar magnetic drivers that have been pleated, like an accordion, so that sound is squeezed out between the pleats).
