Since I work at a non-profit, we are always looking for the best value in apps. A great place to research apps is AlternativeTo.net.
I previously blogged about Pixelmator, which is a fantastic and inexpensive alternative to Photoshop. I discovered it using AlternativeTo.net. I also found Inkscape (free, open-source alternative to Adobe Illustrator), Scribus (free, open-source alternative to Adobe InDesign), and TextWrangler.
It's also good to confirm that you are using the best option and that there aren't better ones out there. e.g. Chrome, VLC, and Skype.