A Google Custom Search Engine, now called Programmable Search Engine is a google toolbar that has been preconfigured to give specific results. 
For example, get the results from a specific site only, a specific type of metada in the page etc.
The positive thing about Google CSE/PSE is that you can configure it to understand pre programmed synonyms to make search far above the 32 words limits. There are also no captchas on CSE and results are different from a normal Google/ Bing search.
A less positive thing, results are limited to 100 per search instead of 300 for a normal search.
Here is a list of CSE we created, just click the link and tap your boolean query in the field.
If you wish to give good CSE to the community, please reach out to us, we'll happily test them and add them (and credit you for it of course).
We are trying to make a documentation for each Search Engine to explain how it works & how to use some custom operators when it's possible (it's a hard work) - If the documentation exists, you just have to click on the logo which is on the left of the search bar.
List of CSE/PSE: 
Dimensions_personnalisées_500x500_px.jp
linkedin x ray results by country.jpeg
aboutme.jpeg
topio.jpeg
crunchbase.jpeg
online CVs.jpeg
linkedin 2.jpeg
github.jpeg
gitlab.jpeg
dsc.jpeg
meetup events.jpeg
researchgate.jpeg
stackoverflow.jpeg
symfony.jpeg
salesforce.jpeg
zoominfo.jpeg
twitter.jpeg
xing.jpeg
g scholar.jpeg
ms dynamics.jpeg
intel.jpeg

And now some very French tools

viadeo.jpeg
meetup events.jpeg
alumnis 15k.jpeg
alumnis rh.jpeg
malt.jpeg
dogfinance.jpeg
jdn.jpeg
copains d'avant.jpeg

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