// The TestFactory will pre-fill all the fields we typically need
Account a = (Account)TestFactory.createSObject(new Account());
insert a;

// You can also set values to be used. Any values set in the constructor will override the defaults
Opportunity o = (Opportunity)TestFactory.createSObject(new Opportunity(AccountId = a.Id));

// You can also specify a specific set of overrides for different scenarios
Account a = (Account)TestFactory.createSObject(new Account(), 'TestFactory.AccountDefaults');

// Finally, get a bunch of records for testing bulk
Account[] aList = (Account[])TestFactory.createSObjectList(new Account(), 200);

// You can optionally insert records as created like this:
// Note the final parameter of true.
Account a = (Account) TestFactory.createSObject(new Account(), true);
Contact c = (Contact) TestFactory.createSObject(new Contact(AccountID = a.Id), true);