I posted a little over a year ago what I considered "essential" OS X apps. This is an updated list.
Web browser: Chrome
On a Mac, you can't beat iTunes, especially if you use an iPhone, iPod, or iPad. It's a solid music program, and they've been making really nice improvements over the past 2 years. Plus the iTunes store is the largest digital music marketplace, so chances are they've got anything you're looking for.
Code editor: Atom
This is a change from last time! The mighty king has been dethroned. Atom is a new editor made by the lovely people at GitHub. It beats Sublime in price too (FREE!). It's lightweight, super customizable, and packed with great features out of the box. Now that it's out of Beta, go get your hands on it!
This one depends. I previously picked Tweetbot because I was using an iPhone, and had Tweetbot installed there and on my iPad. I invested about $30 in the ecosystem overall, between apps for iPhone, iPad, and Mac (the Mac app is $20, so pretty steep). The up side to that was that you had timeline syncing between all devices, so you never had to remember where you were last at. I have to admit, Tweetbot is easily the #1 app that I miss most from iOS, and if I ever switch back to an iPhone it's the first thing I'm downloading. HOWEVER. If you don't use Tweetbot on mobile, I'd suggest the official Twitter app for Mac or just using the Twitter web app at twitter.com. They've added popup notifications from within the website and have recently undergone some nice design changes that make it a pleasure to use. If you like a native app and don't want to shell out $20, stick with the official Twitter app.
Email client: Airmail/Mail.app
This depends what you're using mail for. If you have an Exchange account for work that you want mail on, use Mail.app (Outlook for Mac is painful to use). However, if you want an app for a personal account and/or your company uses Google Apps (I envy you), go get Airmail. It's a super clean, good looking email client that syncs really well with Gmail. It pulls down labels and helps keep you organized, and is a pleasure to use. Airmail is $1.99 and worth the price.
Instant messaging: Adium/Hangouts/Messages
Adium is still an excellent app that can sign in to multiple different IM services. If you need to log in to chat on something other than Google, Adium is the way to go. If you're using Google Chat, get the Hangouts extension for Chrome. It pops up a little chat window that looks and feels native. It even puts a little Hangouts icon in your Mac task bar. If you're an iOS user and want to use iMessage AND Google chat, then use Messages. You can sign in with your Google account and chat with friends within the Messages app. Additionally, with the new version of OS X coming out this Fall, they are allowing you to send SMS texts through Messages, which is really convenient.
App launcher: Spotlight
I previously recommended Quicksilver for an app launcher, but it was too buggy. It wouldn't always start on startup, and the keyboard shortcut didn't always work. Spotlight is built in to Mac, so if you hit command + space bar and start typing an app name, it will show up and you can hit enter to launch it. You can also use it to find files, etc.
Note taking: Simplenote
When I last wrote this article, Simplenote had not come out with a desktop app yet. That has changed. The app is amazing (I'm actually using it to write this post). It's clean and simple, allowing you to focus on quickly jotting down a note or compose an article within a distraction free zone.
To Do: Wunderlist
This has not changed. Wunderlist still has an excellent app for OS X. I've been eagerly awaiting a redesign after an article I read last year, but sadly no news of that lately. If you use Wunderlist or are looking for an app to help with To Do's, check it out.
Disc burning: Burn
This hasn't changed.
If you're burning CD's, use iTunes, but if you need to burn anything else (.iso's to a DVD, data discs, etc), definitely check out Burn. It has a ton of options, is easy to use and understand, and is very reliable (important so that you're not wasting discs on bad burns). Best of all it is free. If you ever burn more than just CD's you should definitely check out Burn.
Read it later: Pocket
This hasn't changed.
Pocket is a great tool that is available for iOS, Android, and Mac OSX (there's also a web app). If you're ever reading through twitter or on a site and see an article you want to read but don't have time right now, Pocket has the answer. You simply save the link to Pocket and next time you open the app, you see a list of all articles you have saved to read.
Terminal: iTerm 2
If you're a power user, you use the Terminal for at least a couple of things. iTerm is an amazing terminal replacement. It gives you tabbed terminal sessions and a very nice full screen mode. It's also free.