NULLIF has been introducde quite a while ago (Oracle 9) but still this function is not well known to the community.

SYNTAX : NULLIF(expr1,expr2)

Nullif compares expr1 and expr2. If they are equal, then NULL is returned otherwise expr1. We cannot specify the literal NULL for expr1.

So where can this function be used?

Eliminiate Division by Zero:

WITH sales (prod, thisyear, lastyear)
        AS (SELECT 'Monitor' ,  50, 25 FROM dual UNION ALL
            SELECT 'Keyboard', 125,  0 FROM dual UNION ALL
            SELECT 'Mouse'   ,  35, 40 FROM dual UNION ALL 
            SELECT 'Desktop' ,   0, 25 FROM dual UNION ALL
            SELECT 'Laptop'  ,  10,  3 FROM dual)
SELECT prod, thisYear, lastYear
     , ROUND(thisYear * 100 / NULLIF(lastYear,0),2) AS SalesPerc
  FROM sales
/

PROD       THISYEAR   LASTYEAR  SALESPERC
-------- ---------- ---------- ----------
Monitor          50         25        200
Keyboard        125          0
Mouse            35         40       87.5
Desktop           0         25          0
Laptop           10          3     333.33

Remove not needed concatenation seperators:

Add first letter of the middle name followed by a dot if a middle name exists.

WITH pers (firstname, middle, lastname)
       AS (SELECT 'Kurt'  ,'Heinrich','Meier' FROM dual UNION ALL
           SELECT 'Hubert',NULL      ,'Huber' FROM dual)
SELECT firstname || NULLIF(' '||SUBSTR(middle,1,1) || '.',' .') 
    || ' ' || lastname AS fullname
  FROM pers
/    

FULLNAME
---------------
Kurt H. Meier
Hubert Huber

Suppress values above (or below) a given margin:

Do not show salaries greater/equal 3000.

SELECT ename, NULLIF(sal,GREATEST(sal,3000)) AS sal
  FROM emp
/    

ENAME             SAL
---------- ----------
SMITH             800
ALLEN            1600
WARD             1250
JONES            2975
MARTIN           1250
BLAKE            2850
CLARK            2450
SCOTT
KING
TURNER           1500
ADAMS            1100
JAMES             950
FORD
MILLER           1300

As you can see there are multiple opportunities to make use of this smart little function.