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

Jetpack Joyride
72

Gold Miner
71

Endless Truck
68

Stickman vs Zombies
81

Fastlane Road to Revenge Online
74

Idle Evolve
57

Burger Clicker
73

Forge of Empires
70

Stormfall: Age of War
71

Battles of Sorogh
65

Tube Clicker
68

KULI
71

Bob the Robber 4
71

Tuber Simulator Online
70

Paper Minecraft
71

Heroic Quest
73

King of Thieves
72

Gone Fishing
73

Poop Clicker 2
68

Bounzy Online
63

Wanderers.IO
64

Road of Fury: Desert Strike
67

Knights Diamionds
69

Mad Digger
69

Ben 10 Upgrade Chasers
86

Police vs Thief: Hot Pursuit game
71

Vikings War of Clans
72

Stickman Shooter
63

X Trial Racing
69

Cyber Hunter
64

Bloons Tower Defense 5
70