FLOW is a diceless rules system developed by Ville Vuorela for his Stalker RPG in 2008. It is also used in Vihan Lapset RPG from Myrrysmiehet Oy.
In the basis of FLOW are challenges: when player characters want to do something that doesn't succeed by just knowing a skill or having an ability, the GM makes it into a challenge. Numerical value is assigned as the challnge level and then the combination of player skills, abilities, roleplaying and description of actions is summed up. If the sum is higher that the challenge level, the characters succeed and if it is lower, they fail. Anything that is easy to do with right skills succeeds automatically based on GM discretion. Without dice the system allows for better focus on the roleplaying in challenging situations instead of just throwing dice.