Try a Video Game Code Book. These code books are every where! But I will give you the one I know:
LB up, RB up, LB LB up, RB RB up. This cheat will give you invinceblity.
Also search the internet, Go to www. Planethalflife.com for offical Half life cheats, answers, and even Half Life theamed toys. You can Also Visit the Half Life Wiki (CombineOverWiki) Hope this helps :D