A popular addon is the ACE toolkit. I'm sure that there are many more that may simply not be available for public consumption.
I do financial modelling and our firm as developed its own formula auditing (and Excel Swiss Army knife) tool. For example, we can analyze a block of cells and have a visual overlay of which cells contain the same formulas, which cells contain formulas of different types (external file, external sheet, embedded constant, constant, etc..).
This lets us really optimize our time while giving is more confidence that we have properly reviewed an entire spreadsheet.
I do financial modelling and our firm as developed its own formula auditing (and Excel Swiss Army knife) tool. For example, we can analyze a block of cells and have a visual overlay of which cells contain the same formulas, which cells contain formulas of different types (external file, external sheet, embedded constant, constant, etc..).
This lets us really optimize our time while giving is more confidence that we have properly reviewed an entire spreadsheet.