ExcelDNA

ExcelDNA (via hubFS) allows custom Excel functions to be written in .NET with zero deployment effort. In F#, it's as simple as defining a library file of annotated functions, wrapping it in some XML .dna file (where $SCRIPT is replaced by the content of the actual F# script), and copying the ExcelDNA.xll to give it the same name as your .dna file. Then, double-clicking on the .xll file opens up Excel augmented with the custom functions. It's excellent.