Split date range into one row per month in sql server

This is leap year safe and handles date ranges the other answers currently don't.

DECLARE @d TABLE(from_date DATE, to_date DATE);

INSERT @d VALUES ('2013-11-25','2013-12-05');

;WITH n(n) AS 
(
  SELECT ROW_NUMBER() OVER (ORDER BY [object_id])-1 FROM sys.all_columns
),
d(n,f,t,md,bp,ep) AS 
(
  SELECT n.n, d.from_date, d.to_date, 
    DATEDIFF(MONTH, d.from_date, d.to_date),
    DATEADD(MONTH, n.n, DATEADD(DAY, 1-DAY(from_date), from_date)),
    DATEADD(DAY, -1, DATEADD(MONTH, 1, DATEADD(MONTH, n.n, 
      DATEADD(DAY, 1-DAY(from_date), from_date))))
 FROM n INNER JOIN @d AS d 
 ON d.to_date >= DATEADD(MONTH, n.n-1, d.from_date)
)
SELECT original_from_date = f, original_to_date = t, 
  new_from_date = CASE n WHEN 0  THEN f ELSE bp END,
  new_to_date   = CASE n WHEN md THEN t ELSE ep END 
FROM d WHERE md >= n
ORDER BY original_from_date, new_from_date;

Results:

original_from_date   original_to_date   new_from_date   new_to_date
------------------   ----------------   -------------   -----------
2013-11-25           2013-12-05         2013-11-25      2013-11-30
2013-11-25           2013-12-05         2013-12-01      2013-12-05

SQLFiddle demo with longer date ranges and leap years

If you are operating in a dimensional data warehouse, utilize the date dimension. Otherwise, use CTE.

WITH cte AS
(SELECT from_date
      , to_date
      , from_date AS mo_from_date
      , DATEADD(day, day(from_date)* -1 + 1, from_date) AS bom_date
   FROM DateTable
UNION ALL
SELECT from_date
     , to_date
     , DATEADD(month,1,bom_date)
     , DATEADD(month,1,bom_date)
  FROM cte
 where DATEADD(month,1,mo_from_date) < to_date
)
SELECT mo_from_date
     , CASE when to_date < DATEADD(month,1,bom_date) THEN
           to_date
       ELSE
           DATEADD(day, -1, DATEADD(month,1,bom_date))
       END AS mo_to_date
  FROM cte