What are some alternatives?
When comparing Counter-Strike and DotA 2, you can also consider the following products
Xonotic - Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux.
Heroes of Newerth - Heroes of Newerth is a Fantastic, Action, Multiplayer Online Battle Arena, Fantasy, Strategy, and Multiplayer video game created and published by Frostburn Studios.
Battlefield - The latest news and updates for all things Battlefield, the EA and DICE video game franchise.
League of Legends - League of Legends is an Entertaining, Action-Adventure, MOBA (Multiplayer Online Battle Arena), Online and Multiplayer video game developed and published by Riot Games.
AssaultCube - AssaultCube is a FREE multiplayer first-person shooter game. Taking place in realistic environments, with fast, arcade gameplay, it's addictive and fun!
Quake - Quake is a series of first-person shooter video games released by id Software.