One of my favourite memories of early teens was playing HL1 Deathmatch with my brother. We didn't have a situation that enabled actual Internet play, and we were not "in the know" about metastrategy or anything. Learning to endlessly use the longjump module with the OP4 covert ops character that did flips when jumping, to fly across boot_camp in seconds, was just exhilarating and eventually led to my brother never playing against me again. I'd just fly around the map picking up health/energy/grenades and then just blast him with the grenade launcher and disappear again.

Such good times.