Is there anything like NVELO Dataplex (hybrid SSD/HDD software) for OS X?
This is an old question, but for the benefit of anyone who comes across it: Apple introduced this capability under the name Fusion Drive in late 2012. Officially you can only get a Fusion Drive as a build-to-order option on a new Mac, but many people have reported success with configuring their own using the diskutil Terminal command in OS X 10.8.2 or later.