1. VLC
The Daddy of all media players. Plays almost every media type with a wide variety of options and a minimal UI.
2. GIMP
A popular free alternative to Adobe's popular Photoshop image editing software. Not quite on par with Photoshop and the UI is a little difficult but still a brilliant alternative.
3. Banshee
Personally I've found the range of audio players available on uBuntu to be disappointing with difficult, unfamiliar interfaces to someone so familiar with iTunes. There seems to be an obsession with stuffing as many different windows into one interface - coupled with the inability to remove them (I'm looking at you Rhythmbox)
The exception has been Banshee. Although it arrives with a multitude of different windows, these can be easily turned off (View > Show Browser) and the result is a wonderfully simple, sleek and dependable music player.
4. LibreOffice
The popular free alternative to the Microsoft Office suite. Writer for Word, Calc for Excel, Impress for PowerPoint. I've found just about everything I can do in Microsoft Office I can do in Libre.
5. Truecrypt
Lock your important files, encrypt your drives (although this can take a while depending on size) or create hidden drives within drives for extra security.
