XLL Add-ins

Introduced in Excel 95
These can be browsed to from the Excel add-ins dialog box ?

link - learn.microsoft.com/en-us/previous-versions/office/developer/office-2007/aa730920(v=office.12) 
link - learn.microsoft.com/en-us/office/client-developer/excel/excel-xll-sdk-api-function-reference 

An XLL is only loaded once per session

Registry Keys

If a .xll file has been added to the registry with an OPENx
If the file cannot be found then a message will be displayed saying just that.
Clicking on the entry in the Excel Add-ins dialog box will then prompt you and allow you to remove it.

C++ /CLI

This is Microsoft's managed version of C++.

XLW

XLW wraps the C API described in Microsoft Excel 97 Developer's Kit in a few classes that make API programming with Excel much, much easier

link - xlw.sourceforge.net/ 
link - xlw.sourceforge.net/refman/annotated.html 

Excel4Net

link - excel4net.com 

ManagedXLL

link - managedxll.com 
link - stochastix.com/solutions/excel/managedxll/latest/features 

XLLPlus

link - planettechsolutions.com 
link - planatechsolutions.com/xllplus6-online/dotnet_requirements.htm 
link - as-ltd.co.uk/xllplus/default.htm 

NetOffice

link - netoffice.io/