[mjn]

One angle is selling plugins to popular creative software. Not quite "the old way", since they are standardized components and sold somewhat more like apps are, but it's a way of selling software components. Photoshop plugins, VST plugins for audio applications, and plugins for the Unity game engine are three fairly active markets I know of.

Another strategy, though from what I can tell with declining popularity, is to write a GPL-licensed open-source library, and then sell proprietary licenses to companies who prefer those terms. Two random examples: http://www.juce.com/documentation/commercial-licensing http://www.cgal.org/license.html

[SyneRyder]

I wouldn't recommend making Photoshop Plugins to anyone now, unless you have some absolutely killer must-have algorithm (in which case, you'd probably still make more money by licensing it to Adobe / Google etc). That ship has largely sailed, its heyday was around 2007 or so. But if you executed well, there was definitely money to be made at the peak. (Literally, the peak of the Crossing The Chasm bell-curve.)

(Disclosure, I've been writing/selling Photoshop plugins for over a decade - but I'm mostly working on other projects nowadays.)

[cpeterso]

MAD, the Mars Audio Decoder, is an MP3 decoder library that is available as GPL or commercial license. Macromedia licensed MAD for the Flash Player (where it's still used today).

http://www.underbit.com/products/mad/