There have been occasional Apple pop-ups entirely unrelated to Growl that have been suggesting the Mavericks upgrade, and the EOL of Mavericks. I haven't looked at how those were implemented, nor around the schedule being used for those; whether those were installed by a patch, or if those are being triggered from Apple's servers.
Etrecheck is the usual tool for investigating and listing the plug-ins and extensions. It'll install and run as far back as OS X 10.6, and will provide a list of what sorts of extensions and add-ons have been installed.
If these are the Apple pop-ups, you likely will not find any extensions.
If your system is supported by Mavericks then that upgrade can be appropriate here — do check for PowerPC applications, do some research on the changes, then create one or preferably more external backups, then perform the upgrade.
If you do have PowerPC (PPC) applications, that can mean mean you'll be using Snow Leopard until you can upgrade or can replace and retire those applications; booted either natively or booted via Snow Leopard Server running as a guest in a virtual machine. About This Mac > More Info > System Profiler > Software > Applications will get you the list of applications. The "kind" column will generally show Intel, Universal, PowerPC or maybe Classic, and it's the PowerPC and Classic that'll be trouble, if you depend on those particular applications and there's no upgrade available. (You're very likely not using Classic apps on Snow Leopard — support for those ended with OS X 10.5 on PowerPC systems IIRC, and those apps would not work on OS X 10.6 with Intel, but I did find one lurking on a 10.6 server box that I just checked, hence the mention.)
If you're at a minimal hardware configuration for Mavericks, then I'd look to add memory and storage, but then that's normal — the environments I work with tend to see expanding memory and storage requirements over time.