Zune HD users should be able to download the latest firmware update now, and it sounds like it's substantial enough to warrant grabbing ASAP. The browser has been sped up, predictive text entry (a la iPhone) has been added, and there's support for upcoming 3D games.
That's just a short list of what's available. Download and see for yourself, then report back with anything else new/better you find. [GDGT]