Suppose these are in A1:A1000
Insert a new row 1 and in A1 type a label (could be just the word ZIP)
Select all of the occupied A cells; use Data | Advanced Filter and specify
Unique
Now you have a list of the zips; copy these to A1 of Sheet 2
Select the data in A of Sheet1 again and use Data | Advanced Filter and
specify show all
In B1 of Sheet 2 use =COUNTIF(Sheet1!A:A,Sheet2!A1)
Copy this don the column
OR : take the plunge and learn the magic of Pivot Tables
If you have them in Col A
Then enter a heading in A1 (like Zip, insert a row if required), select Col A
choose Data->Filter->Advanced Filter
Click on Unique values only
enter B1 in Copy to
This will give you list of unique zip in Col B
now in C2 enter
=COUNTIF(A:A,B2)
and copy down till the end of your data to get the count