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

Mad Digger
69

Road of Fury: Desert Strike
67

Fastlane Road to Revenge Online
74

Gone Fishing
73

Paper Minecraft
71

Wanderers.IO
64

Ben 10 Upgrade Chasers
86

Forge of Empires
70

Bounzy Online
63

Idle Evolve
57

Burger Clicker
73

Tube Clicker
68

Gold Miner
71

Bob the Robber 4
71

Battles of Sorogh
65

Cyber Hunter
64

Vikings War of Clans
72

King of Thieves
72

KULI
71

Endless Truck
68

Knights Diamionds
69

Tuber Simulator Online
70

X Trial Racing
69

Stickman vs Zombies
81

Stormfall: Age of War
71

Stickman Shooter
63

Police vs Thief: Hot Pursuit game
71

Bloons Tower Defense 5
70

Poop Clicker 2
68

Jetpack Joyride
72

Heroic Quest
73