The library is a treasure chest filled with free gems for people of all ages, but this link is specifically for parents looking for a a free activity for their preschooler!  Several things to make preschool story time even better are:

1)  Try different branches of the library.  The quality of story hour will vary greatly from librarian to librarian, and there are story times offered nearly every morning and afternoon of the week through out the city.  We go to the Groesbeck branch, and enjoy a great story time without a huge crowd.  My two youngest daughters (age 3 and 5) love it.  

2)  Stay for the entire story time (even if your child can't sit still - you can go out and look for books if they're just too fidgety), because there is generally a craft at the end.  Think of it!  A free craft,  no preparation for you, and no mess in your home!!!  You can't beat that!

3)  Check out some of the books the librarian read.  Usually they will have multiple copies of the book, and they also put the books up front for the kids to check out.  My girls love to take home the books that were read, and then we often sing the songs we sung at storyhour at home when we read the books.  Children love going over and over the same material, and I love it because I don't have to come up with new songs or stories for them!

4)  Take time afterwards to allow your kids to explore the children's section of the library.  Let them pick more books.  Don't limit them to just one or two - a love for reading comes from a love for books.  We usually leave the library with a huge bag full of books for the kids.