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 vs Zombies
81

Cyber Hunter
64

Road of Fury: Desert Strike
67

X Trial Racing
69

Fastlane Road to Revenge Online
74

Jetpack Joyride
72

Bounzy Online
63

Stormfall: Age of War
71

Ben 10 Upgrade Chasers
86

Burger Clicker
73

Wanderers.IO
64

Idle Evolve
57

Police vs Thief: Hot Pursuit game
71

Knights Diamionds
69

Bloons Tower Defense 5
70

Forge of Empires
70

KULI
71

Poop Clicker 2
68

King of Thieves
72

Tube Clicker
68

Battles of Sorogh
65

Vikings War of Clans
72

Endless Truck
68

Paper Minecraft
71

Heroic Quest
73

Gone Fishing
73

Tuber Simulator Online
70

Bob the Robber 4
71

Mad Digger
69

Stickman Shooter
63

Gold Miner
71