This is a multi-protocol instant messaging client, very well integrated to OSX. Check the website for the supported IM protocols, but most likely it has the ones you need (including AIM, MSN, Jabber/GTalk and Yahoo).
Adobe Photoshop Elements 6 [commercial - €90]
Good photo editing software for amateurs and enthusiasts.
Adobe Reader [freeware]
To open PDF files.
aMule [freeware]
emule (edonkey) clone for Mac OS.
Azureus [freeware]
Java-based bittorrent client.
FacebookSync [freeware]
Import Facebook contact details to the Mac OS address book. Alternative download link here.
Filezilla [freeware]
FTP client.
Firefox [freeware]
No need to introduce the leading multi-platform and open-source web browser.
Flickr Uploader [freeware]
Easily upload your pictures to Flickr to share them online.
Flip4Mac WMV plug-in [freeware]
Play Windows Media video files in Quicktime and all Quicktime-based applications (including Frontrow).
Fluid [freeware]
Webkit-based (Safari) site specific browser. It will let you turn web applications (like gmail or gtalk) into a native Mac OS application with it's own icon, window ... and supports scripting to make the application interact with the system (example: display new e-mail notification in growl or in the dock).
Growl
Perrian
Photomatix [commercial - $99]
Generate High Dynamic Range (HDR) pictures from multiple shots with different exposure compensation.
Safari AdBlock [freeware]
Remove ads (pictures, flash0 from webpages in Safari, based on an online URL list.
Spanning Sync [commercial - $25/year]
(enter the coupon code 3TFDVT at registration for a $5 discount)
Synchronize Mac OS' address book and ICal (and by therefore your iPhone) with GMail contacts and Google Calendar.
XiaoCidian [freeware]
This is a Chinese language add-on for the Apple dictionary application. It can be used to display instant translation by typing ctrl+cmd+d from some applications including Safari (first, select a word or put the mouse pointer on it).
