Make an auto-incrementing number appear in the rows of an Excel column within an otherwise consistent entry.

Manually make the first two rows with the values "foo1" and "foo2" while substituting what you need for "foo" itself. Then select the two cells, release the mouse, and move the pointer over the lower right corner of your selection like this:

 
 

Next: Click and drag downwards.

 
 

Finally: Just let go!