There is a perception that Mac OS is very expensive. It is true that the hardware is expensive than other available PCs; but it is also true that the stability of the system compensates for the price over time.
On the software side, there are some great paid apps, like Omnigraffle, but Mac OS developers have also produced some great free applications. Here are the free applications that I use regularly:
Netnewswire: A comprehensive offline RSS feed reader with Google Reader integration. Does one thing and does it well.
Chrome: A great, cool browser from Google.
Notational Velocity: nvAlt is my default note taking application, containing hundreds of notes synced with Simple Note, which in turn syncs with my iPhone, making all of the notes readily available all the time.
Dropbox: The service that made cloud sync as a fashion.
Skydrive: As a user of MS Office for Mac, Skydrive becomes default choice to store and share [MS Office] docs.
Skitch: When I have to annotate images and screenshots of web-pages, I turn to Skitch.
Evernote: This used to be my choice tool for saving notes, but now that I use Notational Velocity, Evernote is only used for rich text notes (the ones with images and tables).
Mindnode Lite: This lite version is sufficient enough for my needs of mind mapping. My needs are simple like the one, I made for book review about The McKinsey Way.
Unarchiever: This little tool understands every compression format including .7z.
Yemu Zip : When Finder compresses a file or a folder, it includes quite a lot of hidden files. Obviously, when it is unzipped by your Windows colleagues, they get confused with all these hidden files. Yemu Zip zips without these hidden files, keeping sharing of files clean.
Skype: Video conferencing from desktop. Seems to work even on lower bandwidth.
Skim: In my profession, I need to read lots of documents, mostly in pdf format. Skim makes it easy to annotate as I read through. It has the feature to create a pdf file with annotations or export annotations separately as text file.
Appcleaner: Uninstalling in Mac is as easy as dragging the app to the Trash. But the configuration and other support files are left behind. Appcleaner ensures all files are deleted.
VLC: Player of almost all video formats available today.
Kindle: e-Book reader from Amazon.
Flux: Tool to automatically adjust lighting of your Mac so that your eyes aren't hurt when working late nights.
Ommwriter: A minimalist full screen writing app if you want to write without any distraction.
Carbon Copy Cloner: Though Time Machine is the default backup utility for Mac, I somehow still stick to Carbon Copy Cloner. I've never tried to restore back from the backups but I do browse through the backup folders and I've found them to be okay.
Sync Two Folders: Carbon Copy Cloner is used for full backup, but Sync Two Folders is used for syncing photos & videos to another set of backup drives. It has features to simulate before real syncing which is helpful when you sync after a prolonged delay.
As web & desktop developer, I also use quite a lot of development tools. Some of the best free tools for software development under Mac are:
Source Tree: Source control client for both Git & Mercurial.
Xcode: Apple's IDE.
Filezilla: File transfer tool.
What are the free tools that are not in this list, that you would recommend?