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

https://na1.salesforce.com/help/doc/en/salesforce_useful_validation_formulas.pdf

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