Dropbox [not just for Mac]—crazy useful service that makes sharing files across multiple computers and mobile devices automatic and painless; also does some other handy things.
skitch—elegant and enormously useful screen grabbing/annotating/sharing utilty; I’ve used it for a couple years now and find it just makes visual communication easier; also has a handy attached web service for hosting images longer-term
1password—best Mac solution I can find for managing web service passwords while employing good security/privacy practices; also useful for storing other kinds of information like software license keys; syncs well with iOS clients (uses Dropbox, above)
Alfred—I am not a fan of the OS X Dock or other Dock-like things. I like Spotlight in principle, but not in practice. I like Alfred. I use it to launch apps, to initiate various web searches, and for its clipboard history feature. All without lifting my hands from the keyboard. Alfred also has many other features, but these are the ones I use.
Things—I’m a reasonably disciplined person, but I juggle more projects than I can keep track of well. So I use Things. Things is an extremely well thought-out and executed tool for Mac, iPhone and iPad. They all sync together. It’s basically a to do list manager with projects and various reasonable and handy features. But not too many features. Things is ostensibly designed for practitioners of Getting Things Done (GTD). I am not a GTD devotee, but I do like and use Things. Every day.