Artsgraph (this is by far the most comprehensive specialized scientific and math calculator you can get for HPCs)
 

Bandwidth Calculator (performs one function...calculates bandwidth)
 

BizCalc (fantastic HP-12C emulator)
 

BowlingCalc (keeps track and figures scores for up to 4 players...nice)
 

DynaMetric (many conversions, especially unique for physics calculations)
 

KSE Abacus XT (nice solid calculator)
 

MBA-Calc (MBA-Calc operates like the Texas Instruments BA-II Plus calculator )
 

MCalc v1.1 (floats on top of any application and can be moved around,  I prefer these types)
 

Calc98 (many options, many built in calculators not shown, very powerful...in the top 3 for sure)
 

CalcCE (simple desktop replacement calculator)
 

Conversions in Hand (multipurpose convertor)
 

CoolCalc (can be configured with whatever text, buttons and formulas you want, many skins already done)
 

MicroCalc (another free floating calculator, simple easy to see and use)
 

PercentCE (does one thing, calculates percentage of a number from a whole)
 

Proficiency (multifunction calculator with many options)
 

SWEuro (exchange rate calculator)
 

TascalCalc (nice  calculator with many options)
 

TascalGuage (conversion calculator)
 

Yeldnats CF convertor (convert between fahrenheit and centegrade)
 

Yeldnats Convertor (conversion program)