No one sees them coming. While the rest of the summer movie audience is distracted by booming Dolby bass, mile-high billboards or enough Happy Meal tie-ins to choke a horse, these movies sneak in under the radar and clean up at the box office. Usually, they lack super-stars or mega-budgets. Instead they count on heart, laughs and whatever kind of good will they can generate. While it's too soon to say if "Knocked Up" will join this pantheon of stealth performers, it sure looks promising. Here we present a look at some of its forebears and how they fared in summers past.

(Suzanne Hanover / Universal Pictures)