Video Player / Codec
In Windows, we have things like K-Lite Mega Codec Pack that enable us to play almost anything using Windows Media Player / Media Player Classic; In Mac OS X, you have Perian, which is like the K-Lite Mega Codec Pack. After installing Perian, you can basically play anything using Quicktime Player. (Quicktime Player is just like Windows Media Player under Windows).
If you don’t like to use the Quicktime Player, you can try other video players like VLC or Real Player. VLC can handle almost any video format except for things like RM and RMVB.(Real Player can handle these).
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Photo Editing
If you are just doing it for a few casual occasions, then iPhoto would be more than enough for what you would need. However, if you want to do some professional work on your photos, you might want to check out Lightroom (from Adobe, also available for Windows) and Aperture (from Apple). Both of them have a 30 day trial version so you can try them out and decide on the one that suites you best. Last but not least, you have the good old Photoshop that most of us probably have used before 
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Video Editing
Just like above, if you are just doing it for a few casual occasions, then iMovie would be more than enough for most of us. If you want to do more on your videos, you might want to check out things like Final Cut Express HD (from Apple) or even Premiere (from Adobe). If you are a professional film maker, you might be looking at things like Final Cut Studio 2 (from Apple).
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Virtual Machines
At times you might want to run Windows / Linux / <insert more OS here> along side your Mac OS X. The two biggest products on the Mac platform right now is VMware FusionParallels Desktop for Mac and . Both offer great functionalities and you should really try them out before deciding which one to use.
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Web Browser
Safari is the built-in browser for Mac OS X (just like Internet Explorer under Windows). Its a very fast and elegant browser. If you want more functionalities, then you might want to give other browsers a try. Apart from Safari, you can use Opera, Firefox, Camino, Netscape etc. I personally like to use Firefox because I like to make use of the Add-ons. If you just want a fast browser , then Safari should be able to satisfy all your needs.
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Want to know more about Mac software?
Check out the software download page on apple.com
