Attempting to launch an expired game will trigger this prompt: "This content can be used if you renew the license in PlayStation Store." You'll be able to launch directly into the PlayStation Store, where you will be able to purchase the expired title. For example, we could purchase Aero Racer for its full price of $2.99 from the PlayStation Store. We could also choose to purchase a new PlayStation Plus subscription to extend the license.
Curiously, PSone games (like Syphon Filter) appear to work even after the Plus subscription ends. Although clearly marked as Expired under the game's "Details," the game launches normally. It's unclear if this is simply an oversight, or if this content is meant to be free, even after the end of a subscription (much like downloaded avatars and XMB themes). Nevertheless, we've asked Sony to clarify because, hey – permanently free games makes PlayStation Plus a much more palatable offer! If it's a glitch, you can look forward to another mandatory firmware update sometime soon (sorry!).