xCELLencrypt

xCELLencrypt is a lightweight add-in for Microsoft® Office Excel® 2003, Excel® 2007, and Excel® 2010 that allows you to encrypt the contents of individual cells within a worksheet.

The software implements advanced encryption standard (AES) 256 bit encryption via Microsoft's® cryptographic service provider (CryptoAPI).
xCELLencrypt is great for individuals or organizations that share spreadsheets but need to protect certain data within the workbooks. Some examples are: employee rosters that contain sensitive data such as social security numbers, salary data, etc.; personal information like bank account numbers, user names/passwords, security questions and answers.