BetterTouchTool, which is used to enhance gesture by trackpad (I basically use it to flick from spaces to spaces and trigger Exposé). You can also buy it from the App Store if you wish to fund the author.
CloudApp, a Dropbox challenger. A little bit different, though, it’s more about quick upload than managing a directory. With a simple shortcut, you’ll upload a selected file, and they’ll directly paste to the clipboard a short link. The biggest drawback for now is that this link is a link of a viewer, and you must go there to pick the direct link, which is not convenient. By the way, it also lets you know by clicking the menubar icon how many views an item has reached.
Dropbox, Apple should have done it.
Evernote, I use it sometimes, to store ideas that come to my mind, quotes I hear, it’s quite useful, but it probably needs to be more organized. But it’s free, and communicate from iOS to OS X, and the search engine is perfect.
gfxCardStatus, this little utility lets me choose which graphic card I want to use (Nvidia or Intel Chip), and trigger the behavior (I can activate one of them, or both like the original behavior).
Next to the Wi-fi icon, MenuMeters helps me to monitor my RAM use and CPU load (so I can see numbers dramatically increase when I launch Chrome and it displays Flash content). Next to it, it’s MobileMe’s sync icon.
And there is, of course, a launcher, but it’s invisible. I use Quicksilver, because my only need is to quickly launch apps, and I don’t like Alfred’s focus (when you use Alfred you lose your focus, it doesn’t act like a bezel, but like a separate app, and I don’t like this). Oh, and the right part is written in Mandarin Chinese, and as I’m French, I use my Taiwanese Mac with the American layout, so I can write French and English without missing a single character or accent, and switch to zhuyin when needed :). (From formose)