Here is the link to cheat sheet for some useful validation rules that you can incorporate in Salesforce.com.

Examples of Validation Rules

Some of the validation rules that I found useful are

    • Postal code is in correct format if the country is Canada or US
    • State is valid if the country is Canada or US
    • Country is a valid 2 letter ISO code
    • Number entered is even or odd number
    • Number entered is a multiple of another number
    • Phone number is in (999) 999-9999 format
    • Date entered is a weekday or weekend
    • Date entered is the last day of the month
    • Conditionally require a field based on the value of other field
    • Only the record owner can make changes to a particular field
    • Cross object validation rules