For the mobile version there's actually a very awesome mod: Tapatalk although the iPhone app is $2.99 :\ Still a good one nonetheless!

Some that don't necessarily need to be added but would be a nice touch:

• Article Forum
• vBExperience
• Tweet Poster - This one might actually be something more for the marketplace; Perhaps setting up a TFL twitter account that will have posts for latest items sold in marketplace. That way if a user is following the TFL account they can see immediately when something is added.
• Post Thank You - I'd rather use this to thank someone in a forum than just spamming it to say "thank you" in a post.
• Engage Twitter
• Staff Tracker

Some/most of these are for the most recent version of vB though so I'm not sure how that would pan out if any of the ideas were implemented.