SPARSE Column in sqlserver 2008

Sparse columns are ordinary columns that have an optimized storage for null values. Sparse columns reduce the space requirements for null values at the cost of more overhead to retrieve non null values. Consider using sparse columns when the space saved is at least 20 percent to 40 percent. Sparse columns and column sets are defined by using the CREATE TABLE or ALTER TABLE statements.

Columns with excessive null values can be defined as SPARSE columns.

Needs 4 bytes extra for non-null fixed length data type value and 0 for null.
So, there's a tradeoff.

The smaller the data type,  the more no. of null values required to save space.

Properties of Sparse Column:-

geography	text
geometry	timestamp
image	user-defined data types