Ehcache
I think this was used to be called Easy Hibernate Cache and Hibernate people promoted it's use. Now it's owned by Terracotta. I used this quite for a while, it's light, fast and mostly easy to use. However it didn't worked that good as a distributed cache environment.
It has multicast support for peer discovery which works pretty well for most of the time. Except it didn't worked on our prod environment where we have 10 nodes across the network. Some of the nodes just didn't connect to each other which is unacceptable.
It also has manual peer discovery, which needs different ehcache.xml's which would point to other nodes. I think you could tell hibernate, which xml to use at persistence.xml file.
Still I had hard time tracking which node joined to the cluster and which failed to.
Memcached
Main reason I tried memcached was, its use at facebook, wikipedia and such. Memcached is designed a bit different from the ehcache. This is written in 'C', had to be build and run as a daemon process. Cache clients can connect to it through telnet and perform. First problem I ran into was building it. Although It build easily on a Linux environment It required some editing to headers on a HP-UX in order to compile. Second and greater problem is, it doesn't have support for Hibernate. I used the library here http://code.google.com/p/hibernate-memcached/ which is not being actively developed anymore. It also has some nasty bugs...
Hazelcast
Hazelcast is what I use now. At first look it's design looks similar to ehcache but this one is made distribution in mind. It's distributed by default. I have enabled back-up reads, and near-cache settings for better performance.
It comes whit a decent monitoring app. which you can monitor your cluster and see the active nodes.
It's much easier to configure especially compared whit ehcache.