In nopCommerce 1.90, by default, the password is hashed with the SHA1 algorithm by the method CreatePasswordHash() in the CustomerService class (file: Libraries\Nop.BusinessLogic\Customer\CustomerService.cs). The hash value is of the plain text password and the salt.
For 1.80, the method is the same but is located in file: Libraries\Nop.BusinessLogic\Customer\CustomerManager.cs -the default hashing algorithm is also SHA1 (setting: Security.PasswordFormat)