Commercial open-source: Sentry
Commercial open-source software is usually based around some kind of asymmetry: the owner possesses something that you as a user do not, allowing them to make money off of it.
This asymmetry can take on a number of forms. One popular option is to have dual licensing: the product is open-source (usually GPL), but if you want to deviate from that, there’s the option to buy a commercial license. These projects are recognizable by the fact that they generally require you to sign a Contributor License Agreement (CLA) in which you transfer all your rights to the code over to the project owners. A very bad deal for you as a contributor (you work but get nothing in return) so I recommend against participating in those projects. But that’s a subject for a different day.
Another option for making asymmetry is open core: make a limited version open-source and sell a full-featured version. Typically named “the enterprise version”. Where you draw the line between both versions determines how useful the project is in its open-source form versus how much potential there is to sell it. Most of the time this tends towards a completely useless open-source version, but there are exceptions (e.g. Gitlab).
These models are so prevalent that I was pleasantly surprised to see who Sentry does things: as little asymmetry as possible. The entire product is open-source and under a very liberal license. The hosted version (the SaaS product that they sell) is claimed to be running using exactly the same source code. The created value, for which you’ll want to pay, is in the belief that a) you don’t want to spend time running it yourself and b) they’ll do a better job at it than you do.
This model certainly won’t work in all contexts and it probably won’t lead to a billion dollar exit, but that doesn’t always have to be the goal.
So kudos to Sentry, they’re certainly trying to make money in the nicest way possible, without giving contributors and hobbyists a bad deal. I hope they do well.
More info on their open-source model can be read on their blog: Building an Open Source Service.