The nice thing about Mac's is that there are tons of developers making awesome applications that you can use on your Mac. And the great thing is that most of them are free, or very inexpensive. I am an avid Mac user, and I have used hundreds of applications. I have compiled a list of all of the best apps that I have found, hopefully you find them useful as well!
Quicksilver is one of the easiest application launcher's out there. Its so convenient you can set the shortcut key to launch it, and easily start typing the application you want to open, and thats it! It scans your applications and files, so that when you add anything new, they will launch through Quicksilver. The software is free, but the company survives on donations.
Using pathfinder has changed my life. I love that I can easily customize the columns on the sides of the application, as well as show my drives, favorites, and applications running all in one place. Its truly a Mac Finder on steroids. Try it out for 30 days with the demo, and I know you will love it. Its worth the price.
Producteev is not your average to do list organizer. You can create multiple lists, and share those lists with friends, have it on your mac and your phone, and each task is very customizable. I personally have 6 lists that I use, one for my house, one for my daughter, husband, work, etc. This way I keep it all organized, I know exactly what needs to be done and when, and I can share the tasks with my husband so that he can help me accomplish some of them.
I personally use 1Password every day. I use it to store all of my passwords, my credit cards and banking information, serial numbers for software that I purchase, and other secure information. My favorite thing about this app is that it integrates with your browser (Firefox, Chrome or Safari), so that you can autofill your usernames and passwords to any website you have saved, and you can even autofill your credit card info! Its so handy to have all of my information all in one place, either on my laptop or my phone. Plus, now I don't have to remember all of my different passwords. I just need to remember one master password, and thats it! I highly recommend this app - it will save you so much headache from having to remember what username and password you used on the site you're visiting.
Skitch makes it so easy to edit a PDF, image or screen shot, allowing you to add your own notes, point out certain things, or just highlight something. I like that it gives you the flexibility to do multiple things, rather than just add text. I have found that its perfect for adding notes to documents, web design mocks, or just adding funny notes to pictures.