Since version 11.04, Ubuntu Software Center has a nice feature to allow users to rate and review the applications. It helps other people to find out which are the best available applications for their need. Besides the default applications, here is the list of the 9 applications that are not preinstalled but get the highest rating scores in Ubuntu Software Center. And even if you are not a Linux user, this post will still be very useful for you because some of these applications can be used in Windows and MacOS as well ( and of course, totally free).
1- Chromium Browser.
Chromium is the original open source idea on which Google Chrome is based. Though you can easily download and install Google Chrome in Ubuntu, I still recommend you to choose Chromium instead.
2- GIMP ( GNU Image Manipulation Program)
Being nicknamed as the Photoshop for poor guys, but GIMP is more than enough to meet all of your needs if you are not a professional photographer or designer. Moreover, with the better looking single-window mode and many new features in version 2.7.3, GIMP is surely a very good application for you. And if you are using Windows or MacOS and dont have money to buy the expensive Adobe Photoshop, GIMP will be your choice since it's totally free. Download GIMP for Windows and MacOS here.
3- VLC Media Player
VLC is written by the VideoLAN project. It supports many audio many audio and video codecs and file formats as well as DVDs, VCDs, and various streaming protocols. In other words, it's really hard to find a media file (not broken of course) that VLC cannot play. VLC is also available and free to download for Windows and MacOS. Download VLC here.
If you need an application to edit audio files, Audacity is always the best choice for you whether you are using Linux, Windows or MacOS. Audacity has many useful features and is always free. Download Audacity for Windows or MacOS here.
Another graphic editor dedicated to do vector graphics designing. It's also free and available for Windows and MacOS. Download Inkscape for Windows and MacOS here.
Perhaps the best video editor for Linux. It provides a stable, free and friendly environment to edit video. Too bad that you cannot use Openshot on Windows or MacOS.
Though replaced by Banshee as the default music player in Ubuntu 11.04, Rhymthbox still has the favor of many users. It still one of the best music players with many cool features.
8 - Calibre
If you have a huge collection of ebooks, Calibre is a must-have application, it helps you organize, save and manage ebooks, supporting many ebook formats. Calibre also supports e-book syncing with a variety of popular e-book readers like the Amazon kindle. If you are using Windows or MacOS, download Calibre here.
9- Pidgin Instant Messenger
Though the Canonical guys choose Empathy as the default chat application for Ubuntu, Pidgin will always have my vote for the best Linux messenger application.