This classic aromatic seasoning is used primarily for seafood  dishes, but is used in some other dishes as well.  Some interesting history of this spice at Old Bay.  Here's three versions; one made with whole spices that you can grind yourself, and two that are quicker, made from pre-ground spices.  

To buy ready-made, look for McCormick Old Bay Seasoning, frequently with the seafood instead of with the spices.  They bought the rights and the recipe from the original owners.  Frequently you can find it near the seafood counter rather that with the spices.
Ingredients:  CELERY SALT (SALT, CELERY SEED), SPICES (INCLUDING MUSTARD, RED PEPPER, BLACK PEPPER, BAY [LAUREL] LEAVES, CLOVES, ALLSPICE [PIMENTO], GINGER, MACE, CARDAMOM, CINNAMON), AND PAPRIKA.

Here's some homemade versions.

Old Bay Seasoning #1 
Made from whole spices

1 Tablespoon celery seed
1 Tablespoon whole black peppercorns
6 whole bay leaves
1/2 teaspoon whole cardamom
1/2 teaspoon mustard seed
4 whole cloves
1 teaspoon sweet Hungarian paprika
1/4 teaspoon mace

In a spice grinder or small food processor, combine all of the ingredients.  Grind well.  Store in a small glass jar.

From the book "Sundays at Moosewood Restaurant".

Old Bay Seasoning #2
Made from ground spices

1 Tablespoon ground bay leaf
2-1/2 teaspoons celery salt
1-1/2 teaspoons dry mustard
1-1/2 teaspoons black pepper
3/4 teaspoon ground nutmeg
1/2 teaspoon ground cloves
1/2 teaspoon ground ginger
1/2 teaspoon paprika
1/2 teaspoon cayenne pepper
1/4 teaspoon ground mace -- optional

Combine all ingredients and store in air tight container.   

Old Bay Seasoning #3
Made from ground spices

1 Tablespoon celery salt
2 teaspoons black pepper
1 Tablespoon ground bay leaf
1/2 teaspoon ground cardamom
1/2 teaspoon dry mustard
1/4 teaspoon ground cloves
1 teaspoon paprika
1/4 teaspoon mace
1/4 teaspoon cinnamon

Combine, mixing well.  Store in a cool place in an airtight jar. web tool