Find the smallest unused number in SQL Server

Find the first row where there does not exist a row with Id + 1

SELECT TOP 1 t1.Id+1 
FROM table t1
WHERE NOT EXISTS(SELECT * FROM table t2 WHERE t2.Id = t1.Id + 1)
ORDER BY t1.Id

Edit:

To handle the special case where the lowest existing id is not 1, here is a ugly solution:

SELECT TOP 1 * FROM (
    SELECT t1.Id+1 AS Id
    FROM table t1
    WHERE NOT EXISTS(SELECT * FROM table t2 WHERE t2.Id = t1.Id + 1 )
    UNION 
    SELECT 1 AS Id
    WHERE NOT EXISTS (SELECT * FROM table t3 WHERE t3.Id = 1)) ot
ORDER BY 1

No mention of locking or concurrency in any of the answers so far.

Consider these two users adding a document at nearly the same time:-

User 1                User 2
Find Id               
                      Find Id
Id = 42               
                      Id = 42
Insert (42..)  
                      Insert (42..)
                      Error!

You either need to: a) Handle that error and go around the loop again looking for the next available Id, OR b) Take a lock out at the start of the process so only 1 user is looking for Ids at a particular time

If you sort them by numeric ID, the number you are looking for will be the first one for which the ROW_NUMBER() function doesn't equal the ID.