blood packs

A trick I've used that does a nice steady-bleed wound is to soak a small sponge in blood, seal it in a ziplock bag, and punch a lot of little holes in the bag. This can be then be taped to the actor's body in the required position. When it's time to reveal the wound, have the actor clamp their hand over the spot and apply pressure. Once pressure is applied to the sponge it'll start forcing blood out the holes in the bag and run through the actor's fingers. The actor can easily control the flow of blood, simply by varying the pressure. The only downside being that the bag will leak slightly over time, so it's an effect that needs to be used almost immediately (as opposed to a sealed "splatter" bag).