"Conditional" field layout - hiding or showing fields based on another field
The feature request for this has been very popular since day one, so hopefully it's something P&amp;T adds to the core some day.

For the interim I just published a small plugin called Reasons, which will add simple field/value based conditionals to entry type field layouts - pretty much exactly like the OP describes it.

Download from GitHub: https://github.com/mmikkel/Reasons-Craft

Edit
P&amp;T retired the UserVoice page; here is the new home for the feature request