ChemicalAlia works for Gearbox. She also happens to do some amazing Half-Life themed (though she’s done some for other Valve games as well) drawings.
You might know her, as she created that infamous fancy Headcrab Zombie portrait we featured in one of our Roundups a while back.
She’s created a thread in the Episode Two Steam Forums, where you can share any Half-Life fanart you’ve created, or any you’ve found online (be sure to give credit to the original authors). It also contains a dump of some of her own drawings.
Check it out, and perhaps take a look at her DeviantArt page.