Steel Storm - A TPS Game For Windows, Mac and LInux

Steel Storm is an old school TPS game with score oriented competitive gameplay, and is designed for people who like fast paced action and have limited amounts of time to play computer games.

The events take place in an alternative universe where you control an advanced hovertank, packed with the most advanced and sophisticated weapons. Your task is simple, but, nevertheless, is not trivial. In the fight with presumed extraterrestrial invaders, you must prevail!  (via game's page) 

The beautiful graphics, the background music is also good, and it's simple to get start, just use WASD keys and mouse to complete control. In a word, it is a nice game for killing time.

How To: Change Alien Swarm To FPS Shooting

Alien Swarm is a free top-down shooting game by Valve Corporation.  Alien Swarm Can Be Played as FPS with a few simple commands from the developer console.

 Here is how to

Start game and go to Options > Keyboard > Enable developer console.
Then hit ` (to the left of 1) and type: firstperson asw_hide_marine 1 asw_controls 0
This could let Alien Swarm become more cool, terrifying and easier to shoot those goddamn things, but it also reduce the player's sight.
 
Below is Alien Swarm in FPS.

Free Top-down Online Shooting Game - Subvein

Subvein is a top-down multiplayer Shooting game. It's tactical, fast-paced and most importantly, fun! You can buy weapons, mutate to develop powerful skills, progress through the ranks in opposing gangs, earn credits by doing missions and more. Subvein combines fast paced action with strategic character development to keep you playing for hours.

Features

• Dominate with over 20 weapons

• Mutate and develop powerful skills

• Unlock missions and special abilities

• Explore countless user-generated maps

• Play with others online for FREE!

alien swarm - Valve's free game

You must have known Valve, It's a famous company by its first product, Half-Life and  its distribution software, Steam. Several days ago, July 19, Valve released a free game - Alien Swarm.

Alien Swarm is a free top-down shoot-em-up by Valve Corporation. It is a remake of the Alien Swarm mod for Unreal Tournament 2004, and it was developed by the original team, which was hired by Valve during the course of the development process.

• Tactical, 4 player co-op action game with a top-down perspective
• Complete game code and mod tools
• Unlock persistent items by gaining levels
• Over 40 weapons and equipment with countless loadout configurations
• 4 unique classes and 8 unique characters
• Matchmaking, Steam Cloud, Steam Stats
• 64 achievements
• Tile-based map generation tool
• Powered by Source and Steam