I love co-op. Even games I didn’t enjoy much alone become great played cooperatively, such as Serious Sam. Knight Games has a great list of RPGs that support co-op. There are more than I thought although he missed out the superb System Shock 2.

When played with others it’s not nearly as scary though it’s fun specialising - I went psi while two other friends went marine and hacker respectively. It was great fun although I discovered the psionic guy gets near zero psi-hypos for the last quarter of the game, strangely (and annoyingly).