Red Daddy-Long-Legs (Leiobunum sp.)

 basic red daddy-long-legs, often with red legs. Lots of species, some have dark marks on backs.

if gray with dark three-part "hourglass" shape on back use European Harvestman 

If daddy-long-legs shaped but neither of these, use Phalangioidae. 

If vaguely daddy-long-legs-y and found wandering use Eupnoi

If you can tell it's a harvestman and not a spider, but not much else, especially if found under rocks, use Opiliones. 

European Harvestman / European Daddy-Long-Legs (Phalangium opilio)

 basic daddy-long-legs in gray with dark three-part "hourglass" shape on back. 

if red (especially with black legs) then Leiobunum sp. instead.

If daddy-long-legs shaped but neither of these, use Phalangioidae. 

If vaguely daddy-long-legs-y and found wandering use Eupnoi

If you can tell it's a harvestman and not a spider, but not much else, especially if found under rocks, use Opiliones.