Here's an NFPA dwelling unit calculator:
http://www.nfpa.org/nec/Resources/Calculators/dwelling.asp

What others have you discovered, and which ones are the easiest and most accurate to use?

http://www.electrician.com offers the following under "Calculators:"

• Raceway Fill Calculators
• Standard OCPD Sect. 240.6 Calculator
• Table 250.66 Calculator
• Table 250.122 Calculator
• Excel 7.0+ Table 310.16
• How to Determine Cable Tension with a Sheave and Scale
• Cable Tray Pulling Tension Calculator with examples
• Wire Pull Calculator with references
• Excel 7.0 Neher-McGrath Calculator for Power Conductors and Thermal Derating
• Multi-purpose Electrical Calculator
• Voltage Drop Calculator
• Standard Calculator