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

Forge of Empires
70

Battles of Sorogh
65

Stormfall: Age of War
71

Idle Evolve
57

Gold Miner
71

Bob the Robber 4
71

Burger Clicker
73

KULI
71

King of Thieves
72

Wanderers.IO
64

Heroic Quest
73

Stickman vs Zombies
81

Bounzy Online
63

Tuber Simulator Online
70

Vikings War of Clans
72

Ben 10 Upgrade Chasers
86

Paper Minecraft
71

Jetpack Joyride
72

Tube Clicker
68

Knights Diamionds
69

Fastlane Road to Revenge Online
74

Cyber Hunter
64

Mad Digger
69

Bloons Tower Defense 5
70

Road of Fury: Desert Strike
67

Poop Clicker 2
68

Endless Truck
68

Police vs Thief: Hot Pursuit game
71

X Trial Racing
69

Gone Fishing
73

Stickman Shooter
63