vSphere 5.5 introduced the vSphere Flash Read Cache that enables the pooling of multiple Flash-based devices into a single consumable vSphere construct called vSphere Flash Resource.
vSphere hosts can use the vSphere Flash Resource as vSphere Flash Swap Cache, which replaces the Swap to SSD feature previously introduced with vSphere 5.0. It provides a write-through cache mode that enhances virtual machines performance without the modification of applications and OSs.
At its core Flash Cache enables the offload of READ I/O from the shared storage to local SSDs, reducing the overall I/O requirements on your shared storage.

Documented maxima with vSphere 5.5:
- Virtual flash resource per host: 1
- Maximum cache for each virtual disk: 400GB
- Cumulative cache configured per host (for all virtual disks): 2TB
- Virtual disk size: 16TB
- Virtual host swap cache size: 4TB
- Flash devices (disks) per virtual flash resource: 8

There is also the Storage Accelerator (CBRC) feature that can be enabled (only) with VMware Horizon View (VDI).
View Storage Accelerator is an in memory (ESXi Server Memory) cache, caching common image blocks when reading virtual desktop images. It is applicable to stateless (non-persistent) as well as stateful (persistent) desktops and is transparent to the guest virtual machine/desktop. It does not require any special storage array technology and provides additional performance benefits when used in conjunction with Storage Array technologies.

http://www.vmware.com/files/pdf/techpaper/vmware-view-storage-accelerator-host-caching-content-based-read-cache.pdf