Upgrade
Almost every game played online for free this way or another includes upgrades. They can relate weaponry, life, armor, deadly power of your hero, and his/her special abilities. Or weaken monsters. But there are such games, which made the upgrade system an inalienable part of the process and made it necessary to make too often. Like, after each level (every several minutes or sooner). This is the case in launching rockets – they should fly high but without improvements, they will simply show the same result. Or like flying through the distance – only improvements will make you get better results without standing still.
Free online games - Upgrade games
Upgrade games

Gone Fishing
73

Forge of Empires
70

Endless Truck
68

Tuber Simulator Online
70

Jetpack Joyride
72

Idle Evolve
57

Gold Miner
71

Police vs Thief: Hot Pursuit game
71

Cyber Hunter
64

Paper Minecraft
71

Stickman vs Zombies
81

Heroic Quest
73

Tube Clicker
68

Fastlane Road to Revenge Online
74

KULI
71

King of Thieves
72

Burger Clicker
73

Road of Fury: Desert Strike
67

Bob the Robber 4
71

Stormfall: Age of War
71

Wanderers.IO
64

Mad Digger
69

X Trial Racing
69

Bloons Tower Defense 5
70

Poop Clicker 2
68

Knights Diamionds
69

Battles of Sorogh
65

Ben 10 Upgrade Chasers
86

Vikings War of Clans
72

Stickman Shooter
63

Bounzy Online
63