It seems that this weekend a number of popular of popular applications for Mac stopped running or were constantly crashing because of expired developer certificates. The list of tools affected by this problem included names like 1Password, Soulver or PDFpen, but only affected the apps that weren't downloaded directly from Apple's App store. In case you were affected, all three previously mentioned applications have been updated and now work as before.
From what I gather, the crashes were caused by a change in the way the app certificates operate on Macs and a relatively new protocol called provisioning profile. Apple is now requiring a valid provisioning profile in order to run an application and that profile requires a valid developer’s code-signing certificate. Just like the pieces of a domino falling, if the code-signing certificate expires the provisioning profile is no longer valid and the application keeps crashing. Since up until now, all the applications already installed weren't affected by the expiration of the code-signing certificate, many developers didn't worry too much about it, but now they were forced to do so.
In case you're the proud owner of a Mac or a MacBook Pro, you may also be interested in reading: "Best tools to keep your Mac organized", "10 +1 Useful Mac keyboard shortcuts that you should know" or "Best Mac Productivity Tools For Power Users".