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

Road of Fury: Desert Strike
67

Stickman Shooter
63

KULI
71

Bloons Tower Defense 5
70

Ben 10 Upgrade Chasers
86

Jetpack Joyride
72

Tuber Simulator Online
70

Gold Miner
71

Knights Diamionds
69

Stormfall: Age of War
71

Bounzy Online
63

Poop Clicker 2
68

Burger Clicker
73

Fastlane Road to Revenge Online
74

Heroic Quest
73

Vikings War of Clans
72

Bob the Robber 4
71

X Trial Racing
69

Battles of Sorogh
65

Forge of Empires
70

Gone Fishing
73

King of Thieves
72

Police vs Thief: Hot Pursuit game
71

Stickman vs Zombies
81

Paper Minecraft
71

Idle Evolve
57

Cyber Hunter
64

Wanderers.IO
64

Mad Digger
69

Endless Truck
68

Tube Clicker
68