You may be aware of the Gatekeeper security feature introduced with OS X 10.7.5. It prevents potential malware from being run inadvertently on your machine by checking downloaded apps for a developer signature. Most modern applets from this site are "codesigned" with my Developer ID which is registered with Apple. You are therefore assured that my apps are not malware. (It would be pretty dumb of me to release malware in any case.)
Apple has updated the way Gatekeeper signatures are recognized on OS X 10.9.5 and Yosemite. Gatekeeper on those systems (and later) will now look for a new version 2 signature, which can only be implemented by a developer on OS X 10.9 and later. The v2 signature is fully backward compatible on earlier systems. But v1 signatures have been obsoleted and will not be recognized on OS X 10.9.5 or Yosemite.
Codesigned applets that were last updated on Mountain Lion or earlier will have a v1 signature.
I've spent the Summer working on updating v1-codesigned scripts and apps so they will be compatible with Yosemite when it arrives in the Fall. However, I haven't yet updated all codesigned applets with a v2 signature.
If you download an applet signed with a v1 signature on OS X 10.9.5 or Yosemite you may see an alert that advises you that the app is from an unknown developer. If so, you can override Gatekeeper security by Right-clicking/Control-clicking the applet's icon in the Finder and selecting "Open" from the contextual menu.
I hope to have all affected apps updated with v2 signatures over the next few weeks in time for the official release of OS X 10.10.
UPDATE: ArsTechnica has more.