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

Idle Evolve
57

Ben 10 Upgrade Chasers
86

Stickman Shooter
63

Poop Clicker 2
68

KULI
71

X Trial Racing
69

Paper Minecraft
71

Forge of Empires
70

Police vs Thief: Hot Pursuit game
71

Tuber Simulator Online
70

Bob the Robber 4
71

Bloons Tower Defense 5
70

Jetpack Joyride
72

Bounzy Online
63

Road of Fury: Desert Strike
67

King of Thieves
72

Wanderers.IO
64

Gold Miner
71

Burger Clicker
73

Tube Clicker
68

Endless Truck
68

Heroic Quest
73

Cyber Hunter
64

Stormfall: Age of War
71

Vikings War of Clans
72

Gone Fishing
73

Battles of Sorogh
65

Fastlane Road to Revenge Online
74

Stickman vs Zombies
81

Mad Digger
69