Open Source License model to charge big enterprises
I have a reasonably successful library that I plan to open source.
However, I would like to charge big enterprise customers who make lot of money using the library a small fee.
How do I go about doing that? What kind of Open Source license should I use? Any examples?
Open source does not currently allow for what you are trying to achieve. Consider Fair Code: https://faircode.io/
Your best option is to use the GNU AGPL or GPL and sell a different license to companies who find those unacceptable. Be prepared to go to court in order to uphold the GNU licenses, companies will violate them a lot. Also be prepared to have companies reimplement your library from scratch in order to avoid the GNU licenses.
Make it public domain. While any major corporation could still use it without you being paid, many don't and will request you provide the same code but under a license their lawyers are comfortable with. SQLite sort of runs on this model -- it's public domain, but the author has been asked by many big corps for licenses and pay for them. They also pay for direct support, mods to the code. The testing harnesses and other aspects of the development and testing of SQLite but which are not part of SQLite can be kept entirely proprietary. I don't know if this model would fit for your library, SQLite may have a very different niche as an embedded database, and frankly it is used everywhere, so not sure if that would work in your case, but it's another option to think about.
Trademarks with a MIT / Apache 2 / BSD style license are starting to be considered one of the better models we know of right now.
Check out https://coss.media and definitely JJ's video call with GitLab founder Sid
There are no open source licenses that do that, at least none that meet either the Open Source Initiative's definition nor the Free Software Foundation's definition:
https://opensource.org/osd
Public domain or GNU AGPL would work, as mentioned by the other comments. I suggest doing either of these. If many companies won't touch the stuff with GNU AGPL, they can still require a license, so you can offer a commercial license, and then there will be the payment for it. Other kind of multi-licensing with GNU AGPL is also possible.