Half Life 2 is arguably the lifeblood of the modding community and for many the fueling passion behind user driven content. With the want for HL3 it was only a matter of time before the fans developed a graphical update mod acceptable to Valve. While others exist, none have preserved and updated the experience as effectively as this.
Have a peek, then download the mod from the Steam store. Beware, it is a 12GB install and does require Half-Life 2 to play.