http://www.txdot.gov/txdot100/
This section of TxDOT's site includes a legacy video (https://www.youtube.com/watch?v=reJm2jAS0jM), a photo gallery (pop-up window) (http://www.txdot.gov/txdot100/#myGallery), a graphical timeline (http://www.txdot.gov/txdot100/docs/timeline.pdf), and a multi-page .pdf called "Dirt Roads to Diverging Diamonds: A Century of Transportation in Texas" (http://ftp.dot.state.tx.us/pub/txdot/centennial/publication.pdf)
I ran across this while doing a designation files lookup; I didn't know TxDOT had anything planned for their anniversary online. Despite a couple of errors here and there on the big .pdf, it's quite an interesting look back, including the very first not-ready-for-the-public state map of planned state highways in 1917. Many routes on it look like they mirror railroad corridors of the time.