Here is a link to an archive that contains a module for adding CODE 128 barcodes to the PDF documents that Magento generates for Invoices, Shipments, Credit Memos. The barcode is of the order number.
You can download the files I used to create the module here. Just extract the files into your Magento directory as they are listed in the archive. Barcoding Invoice-Shipment-Creditmemo.zip
Updated version for working with v1.2.x of Magento Barcoding Invoice-Shipment-Creditmemo.zip
I’ve used this in combination with the shipping integration with FedEx and Endicia I have listed in this group. That way invoices can be printed in bulk or individually with a barcode and users can just scan a barcode at the shipping stations.
The free TTF font that I used came from: http://freebarcodefonts.dobsonsw.com/
I referenced the documentation on that site in order to build the PHP code necessary to calculate the checksums and convert the string correctly for the font. http://freebarcodefonts.dobsonsw.com/Code128Transformation.htm
If you would like to show your support to Brian Dobson for creating the free barcode font, you can send him $10 or more via PayPal using the email address on his contact page. http://freebarcodefonts.dobsonsw.com/contactus.html
Here is the forum thread I posted the file in: Forum Link