I use Picasa. Especially for its good facial recognition.
Have it scan all the relevant folders, and then it will begin finding faces which you can then tag with people. The more you tag particular faces, the easier it finds those same faces on other folders.
In maybe an hour, with thousands of photos, I’ve got every photo tagged with all family members.
But you’re talking about stock photos though.
I think for you, just use a good tag manager.
Let’s face it, with using folders alone, this is basically one “tag”, (i.e. “outdoor shots”) or whatever. But as soon as you want to tag with multiple features like “outdoor, mountain, sunset, sky, forest, lake, evergreen” then folders become useless.
There are many tools for tagging photos, Picasa included. But you can also tag photos in Windows directly, read this: http://windows.microsoft.com/en-us/windows-vista/find-your-photos-faster-how-tags-helped-me-sort-my-snapshots
As long as the photos are not recreated, such as using tools that resize them or convert them to smaller sizes for attaching in emails or whatever, then the tags you set will be preserved. So if you create some tags on a JPG and email to someone else, they will also have those tags set. If you archive the photos on a cloud service, the tags are preserved.
Then later, you can search by tags, sort by them, group them by tags, etc.
Another option is using a so-called document management application. Some of these are pretty robust. They utilize their own internal database to store much more data about your files. For example they can store multiple revisions of files, connecting one photo, for example, to each of its various sizes or renders. For example, in photography, you would have an original RAW file, and then one or more Photoshop masters, and then various renders and sizes. All of these can be grouped as a single collection.
Some of these tools would run on your workstation, some would need a server, some can run as a cloud service. For example look at a program called Daminion as an example. Most of these professional file management apps won’t be free though.
So then, I would start with a robust file tagging app, perhaps Picasa, ACDSee, and some people even use Lightroom as an organizing tool. Or look around for a complete document management solution.