This is mostly a list of Mac applications, but I've included a few Web apps as well.
Alfred : Useful Application Launcher. (When Quicksilver died I switched to Alfred.)
Divvy : Grid-Based Window Manager.
GitX : Power GUI for Git. Note: There are several forks of GitX. Look at the GitHub network graph to see which has the most activity. From 2010 to 2012, I was using laullon's fork because it was the most active. In the summer of 2012, I found a better maintained fork from rowanj. (This is the beauty of distributed open source!)
Sublime Text 3: I started using Sublime Text 3 in early 2013. Now I prefer it over Textmate 2! It is fantastic. The plugin system is really amazing. You can quickly pull in plugins from 'official' repositories as well as Github repositories.
Emacs: I started using Emacs on a daily basis in early 2013. I am comfortable with it now. It is great for Clojure programming. Still, I find that Sublime Text 3 is nicer looking, has better pan-project searching, and is faster. Emacs gets laggy when I enable things like spell checking.
TextMate 2 : I consider it to be tied in the top two for "best overall text editor for the Mac". If you program, use a real editor like TM, Vim, Emacs, Sublime, or similar. If you choose TextMate, get version 2. Bookmark the documentation pages.
DropBox : Transparent File Sync and Backup. Use that link for a referral and get us both some free space. (Caveat: I haven't compared DropBox against Google Drive head to head.)
GitHub : Collaborative Coding, including Source Control, Wikis, Ticket Tracking. It is great for open source projects, but it is overpriced for private projects that only need a small number of collaborators -- for those, use BitBucket.
BitBucket : Collaborative Coding, now with Git and Mercurial support. Perhaps not as slick as GitHub, but still capable. Much more affordable if you want unlimited private repositories with 5 or less collaborators.
OmniGraffle : Spectacular Diagramming and Drawing. OmniGroup builds solid Mac apps.
Homebrew : Mac OS X Package Management. Step aside, MacPorts, you had your day. Use this.
rbenv "lets you easily switch between multiple versions of Ruby. It's simple, unobtrusive, and follows the UNIX tradition of single-purpose tools that do one thing well." I switched to rbenv from RVM on the advice of others and slightly prefer it.
Pixelmator : Awesome & Awesomely Priced Graphics Editing. Forget the 80/20 Rule! Pixelmator gives you the 99/5 Rule: 99% of what you need from Photoshop, at about 5% of the cost.
Scrivener : Mind-Bendingly Great Writing and Brainstorming. This is the best writing tool for research papers, novels -- anything requiring creativity or organization.
Tuples : PostgreSQL GUI for Mac.
Transmit : File Transfer for FTP and Amazon S3.
Handbrake : Video Transcoder. Yes, also DVD ripping.
MongoHub : Mac Native MongoDB GUI. I recommend the fork by fotonauts; the official download version is not as good.
Awesome Screenshot: Capture & Annotate This Chrome plugin lets you capture all or a portion of a web page in Chrome. Export to a file or share.
Time Out : Be Flexible - Remember to Take Breaks! I wish I was motivated to take stretch breaks, but I usually just find it annoying and turn it off.
RVM : Manage Multiple Ruby Installs. (The bizarre cartoon graphics are a special bonus.) I now use rbenv instead.
OmniFocus : Task Management for the Mac. Get Things Done. Great syncing between Mac, iPhone, iPad. Right now, I'm using index cards. There is something nice about having to-do items written out by hand on stacks of index cards instead of on the screen!
Paparazzi : Full Web Page Screenshots - Zero Scrolling. If you try this without an app or browser plugin, you will fail.
Asana : Team Productivity Web App. I gave it a try, but I did not find it amazing. Granted, I often work solo or on small teams.
Potential... what a great word. But think about it. It implies more "potential energy" than "kinetic energy," right? In other words, something that could go far, but hasn't. Yet.
So, I admit, this is a back-handed complement. Bless their hearts, these apps try so hard. I tried to like these but found them lacking. You might like them.
GitHub's Mac App : (Tries to Help) Manage GitHub Repositories. Pros: simple, elegant, looks good. Cons: Simplistic and doesn't do what I really need for source control. Maybe one day I will like it.
SourceTree : (Almost) Useful Git GUI. Pros: It seems to suggest a nice way of working with Git and touts integration with BitBucket. Cons: I prefer GitX, plus SourceTree was buggy when I tried it in May 2012.