GIMP is like Adobe Photoshop, but GIMP is OpenSource, that means you can use the application for free, even for commercial purpose.
Now the popular image processing application GIMP is available in Mac natively. You don't need to install any X11 wrapper. Download GIMP at http://www.gimp.org/downloads/
TextWrangler is a text editor similar to Notepad++. No, notepad++ is not available for Mac.
Picasa: download from Google.Com
Microsoft OneNote - can be downloaded from AppStore
I also use Chrome, Eclipse and Firefox on my Mac.