Nicholas Gaffney recommends 21 apps to make you more effective. My favorite on his list -- though not so new anymore -- is Google Voice, which gives me excellent control over my incoming calls, with visual voicemail, transcription, and text and email notification. For free.
On my list of apps to try is Slideshare. Slideshare lets you share your presentations over the web, assuming you want that, much like YouTube does for video. Here's one of my favorite such presentations, courtesy of Netflix.
What's on your list of apps to try?