I just spent a day adding MemCached to a customer's web application. Incredible what a difference it makes! The great thing about MemCached is that many client libraries are available and caching works with handling user sessions, web page fragments, some web service calls, etc. Depending a lot on the application, MemCached can take a load off of database servers, back end web services, etc. I was also pleasantly surprised by how simple it was to implement (after carefully analyzing the web application itself - gaining an understanding of what should be cached is the tricky part). Good stuff!
I don't think that I will ever deploy a web application again without analyzing it for MemCached use.