The following are MMO - but on a different scale of of a different style you may be used to. Aside from the first one.. you can only play the following so much a day.
You do have to download the client, but it plays well and I like it.
A few web based games that are worth checking out that you can play from work - no install needed:
Space Builder with invasions I am in universe 28
Village Builder with invasions I am on Server 2