This is one of those blog entries just to remind myself how to do this rather than Google it. It’s pretty simple.
Go to Central Admin –> Application Management
Under Site Collections, click on “Configure quotes and locks”
Under Site Lock Information, select the Read Only (blocks additions, updates, and deletions)
That’s it, you’re done. Now when a user will see something like the following from the Site Actions drop down menu. A subset of the complete site actions for a non read only Site Collection: