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

Stickman Shooter
63

Stickman vs Zombies
81

Bounzy Online
63

Endless Truck
68

Bob the Robber 4
71

Tuber Simulator Online
70

Idle Evolve
57

Road of Fury: Desert Strike
67

Fastlane Road to Revenge Online
74

Gone Fishing
73

Bloons Tower Defense 5
70

Forge of Empires
70

Vikings War of Clans
72

X Trial Racing
69

Paper Minecraft
71

Mad Digger
69

Stormfall: Age of War
71

Wanderers.IO
64

Gold Miner
71

Knights Diamionds
69

Police vs Thief: Hot Pursuit game
71

Poop Clicker 2
68

Cyber Hunter
64

Battles of Sorogh
65

Heroic Quest
73

KULI
71

Ben 10 Upgrade Chasers
86

Burger Clicker
73

Jetpack Joyride
72

King of Thieves
72

Tube Clicker
68