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

Knights Diamionds
69

Burger Clicker
73

Stormfall: Age of War
71

Paper Minecraft
71

Mad Digger
69

Cyber Hunter
64

Poop Clicker 2
68

Endless Truck
68

Stickman vs Zombies
81

Vikings War of Clans
72

Ben 10 Upgrade Chasers
86

Forge of Empires
70

Fastlane Road to Revenge Online
74

King of Thieves
72

Battles of Sorogh
65

KULI
71

Wanderers.IO
64

Road of Fury: Desert Strike
67

Tube Clicker
68

Heroic Quest
73

Gone Fishing
73

Tuber Simulator Online
70

Bloons Tower Defense 5
70

X Trial Racing
69

Police vs Thief: Hot Pursuit game
71

Bob the Robber 4
71

Gold Miner
71

Stickman Shooter
63

Idle Evolve
57

Bounzy Online
63

Jetpack Joyride
72