Dota 2: Defense Of The Ancient 2: A Massive Online Battle Arena (MOBA) game by Valve.
It's free to play and is certainly not pay to win.

Biggest Annual Tournament: The International, where the top 16 teams in the world fight it out.
The International 5: Prize Pool over 14Million USD and counting.
The International 4: Winners Newbee, Prize Pool $10,930,698
The International 3: Winners Alliance, Prize Pool $2,874,381
The International 2: Winners iG, Prize Pool $1,600,000
The International: Winners Navi, Prize Pool $1,600,000


Current Tournaments worth paying attention to:
None
The International 2015
TI4 Recap video.
TI winner recap history

Introduction to the 16 teams Competing by SirActionSlacks (Updates as he releases them)
Archon* (formerly NAR)
CDEC Gaming*
Cloud9
Complexity
Ehome
Evil Geniuses
Fnatic (formerly Team Malaysia)
Invictus Gaming
LGD Gaming
MVP HOT6ix*
MVP Phoenix
Navi
Newbee
Team Empire
Team Secret
Vega Squadron*
Vici Gaming
Virtus.pro

*These 4 teams will have a play off where 2 will advance to the main group stage.
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