Sitecore 9: Dynamic Placeholders
I’d be very surprised if you were not using some sort of implementation of dynamic placeholders in your projects already. Most of us have probably been using a variant of Dynamic Placeholders that Nick Wesselman wrote way back in 2011, my personal favourite being Fortis Dynamic Placeholders by Jason Bert. This is also the most requested feature on the Sitecore Developer Uservoice and Sitecore Community Uservoice.
The wait if finally over. Sitecore 9 introduces support for dynamics placeholder implementation within the core product.