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

Mad Digger
69

Wanderers.IO
64

Ben 10 Upgrade Chasers
86

Bob the Robber 4
71

Stickman vs Zombies
81

Idle Evolve
57

Stormfall: Age of War
71

Tube Clicker
68

Jetpack Joyride
72

Stickman Shooter
63

Knights Diamionds
69

Bounzy Online
63

Forge of Empires
70

Endless Truck
68

Heroic Quest
73

Gold Miner
71

Bloons Tower Defense 5
70

Paper Minecraft
71

KULI
71

Tuber Simulator Online
70

Cyber Hunter
64

Fastlane Road to Revenge Online
74

Burger Clicker
73

Poop Clicker 2
68

Police vs Thief: Hot Pursuit game
71

Battles of Sorogh
65

King of Thieves
72

X Trial Racing
69

Gone Fishing
73

Vikings War of Clans
72

Road of Fury: Desert Strike
67