The Steam Mobile app allows gamers to chat with Steam friends, browse community groups and user profiles, view screenshots and user-generated content for their favorite games, learn more about the games available on Steam and put them on a wishlist or purchase them, read the latest gaming news, stay up to date on unbeatable Steam sales, and more.

After spending months developing Steam for mobile platforms iOS and Android, it appears Valve is now almost complete in that development. Steam Mobile, the iOS/Android counterpart of the widely popular Windows/OS X Steam software, is now available for free on the App Store as well as the Android Market.

If you happen to be a fan of Valve’s Portal game series but you don’t have a copy of the original Portal for Mac or PC, you’ll be happy to know it is absolutely free on Steam until September 20th. Valve is giving away the title as a part of its “Learn with Portals” educational program, hoping to emphasize the role Portal plays on educating gamers in physics and critical thinking skills.

Valve has launched five Free to Play (F2P) games on Steam, including Spiral Knights (the only one that’s Mac compatible), Forsaken Worlds, Champions Online: Free for All, Global Agenda: Free Agent, and Alliance of Valiant Arms (AVA). Starting today, each of the five F2P games will offer exclusive in-game content for those who play the “F2P game of the day” on Steam.

What is the secret of togetherness? Mutual respect? Simple love? Or is it something deeper like hotly anticipated games with new multiplayer levels? Depending on how you and your significant other answer that question, I could have some very good news for you! Valve has announced that Portal 2 is now available for pre-order for an April 18 release, with either 10% off the $49.99 retail price, or 20% off a two pack (from the Steam store), allowing you to play with the one you love.

It was silly of Valve to release Left for Dead after its sequel, because if you’ve played any modern video game, you know that the follow up is “the original game, with more of what you liked the first time.” The two games share the same premise, of course; a mismatched group of four survivors trying to make their way through a zombie apocalypse, running and gunning through different terrains, all while trying to find the mythical “safe zone” free of infection.

Stick together. Watch each others’ backs. Heal your buddy. Keep moving. You might called Left 4 Dead 2 a “survival horror” game, because the goal is simply to make it to the end alive. But unlike other zombie games where you creep along dark hallways, conserving every bullet, L4D2 is a run ‘n’ gun experience, where if you stop too long to think about your next move, you’ll be overrun by a swarm of fast-moving zombies. A “cooperative shooter,” L4D2 is about a team of four survivors of a zombie apocalypse, and to survive, you have to act like a team, providing cover fire, healing each other, and sticking together.

There will likely come a point where I’ll cry out, “Enough with the zombies, already!” But that time hasn’t yet arrived, which will surely please Valve and PopCap Games. They’ve announced that a Game of the Year Edition of PopCap’s Plants vs. Zombies is now available via Steam. More relevant to those of us at Appletell, Plants vs. Zombies for the Mac is available via Steam with support for Steam Cloud and Steam Achievements.

Valve Software announced they are going out of business today—oh, I’m sorry, Valve Software proved they are committed to Mac gaming today (you can see how I made my mistake) by announcing the immediate release of Team Fortress 2 via their Steam delivery system. The game, which was released in 2007 (so it’s like it came out yesterday for Mac users) pits teams of players in various multiplayer matches.

Yesterday, Valve released the third round of Steam for the Mac launch titles. There are some big ones this time around, as this week’s collection includes the Half-Life 2 (HL2) series of games (Half-Life 2, Episode One, and Episode Two) and Eve Online: Tyranis. To celebrate the launch of the Half-Life 2 games, the titles are available for 30% off their regular price for a limited time.