Here's a review of top 10 applications for KDE4.
Yakuake - Great terminal application
Yakuake is a very popular Quake-style terminal application which sits in the background unless it's invoked with the (default) F12 global shortcut. It can inherit Konsole's settings and it is probably the best alternative to Konsole. Just like Konsole, Yakuake supports full transparency effects, various colour schemes and backgrounds, middle-click paste and tabs. Definitely an essential tool.
Amarok 2 - Probably the most popular Linux audio player (full review here)
Amarok's port to KDE4 was probably one of the most controversial releases in the last few months and it generated many discussions whether Amarok 2 took the right way. My answer: yes. Although Amarok 2 was initially stripped by major features (to mention a few: lack of equalizer, currently poor functionality), but instead it is a new ground for what comes to be. It's true, Amarok 2.0.2 was effectively full of bugs (I'm sure not even the devs can argue against this), but the upcoming 2.1 release includes a new, customisable playlist which should be OK. Amarok 2 still misses a lot of features, but the good thing is that developers are aware of the needs of users and plan to implement them in future versions. Among the new features this wonderful player comes with are widget support, a completely rebranded playlist and interface (if you ask me, I like the how the space is managed in the interface), support for many services like Last.fm, Magnatune, and several other music stores. Development advances at a fast pace and pretty soon I'm sure Amarok 2 will be what Amarok 1.4 was and even more.
K3b - Arguably the best KDE burning application (full review here)
Even if it is currently in alpha, K3b has a port for KDE4 now. No new features available and the interface hasn't changed, but this is the ground for what is to come next. K3b is probably the most powerful and full-featured, open-source CD/DVD burning application on Linux.
K3b 1.65.0 alpha - first KDE4 port
SMPlayer - Full-featured video player using Qt
Now that VLC comes with a Qt interface too, I guess this puts SMPlayer and VLC at the same level, but I opted for SMPlayer because I believe it has a more compact interface and it seems to me to be easier to use.
Although SMPlayer does not actually depend on KDE4 libraries, it's still a Qt 4 application, and one of the most powerful video players out there. Since the default Kubuntu video player is Dragon Player, which is not quite configurable and does not have too many features, an alternative must be installed, and SMPlayer does a great job. Using the mplayer engine, SMPlayer is a very powerful, feature-rich and highly configurable video player, which can play any format around, including DVD ISO images or Matroska MKV videos. One of the great features SMPlayer has for a long time now is the ability to remember all the settings for a specific file (like the time position, video settings, window size and position etc). Very useful when you have to close the player, and then later you have to restart a certain video without the need to scroll to the last position in the video. SMPlayer has an interface which fits well with KDE4 and it allows to configure the shortcuts, it provides icon themes, subtitles support, video functions like rotate video, aspect ratio, or various filters.
digiKam - Photo management application
digiKam is a popular photo management application which constitutes the perfect alternative to the closed-source application from Google, Picasa. Starting with version 0.10 it also has a KDE4 port too. digiKam organises your pictures and photographs into albums and collections and comes with a huge number of options and support for many digital cameras.
Krusader - Powerful file manager
With a funny codename, Krusader 2.0 'Mars Pathfinder' was released on April 11 and it is the first Krusader version for KDE4. Krusader was always the first alternative to Konqueror back in the KDE3 days, and its twin-panel interface and powerful features can please any user looking for an advanced file manager for KDE4.
KTorrent - Full-featured BitTorrent client (full review here)
A while ago I put up a full review of KTorrent for KDE4 here. KTorrent has come a long way and is a completely full-featured BitTorrent client, including various useful plugins, torrent information, rich configuration settings, ability to download only certain files in torrents, possibility to create your own torrents and much more. If you are a KDE user and you didn't give it a try, you surely need to.
Gwenview - image viewer and editor
Gwenview is in a continuous development, with more features and improvements added with every new release. Plans for the upcoming 2.3 release of Gwenview include a redesigned places and history handling, which will now include an 'URL bag' (or 'recent URLs' as it will probably be called), so that whenever you start Gwenview to view an image on the web it will keep the address in the history for fast access afterwards.
Gwenview supports various image formats, has good KDE integration, it comes with a file browser and thumbnail previews. One of the features of Gwenview is the ability to use basic image manipulation plugins, which extend its functionality too.
Kdenlive - wonderful video editor
Linux has several popular video editors praised by some and hailed by others, and those include Kino and Cinelerra. A newer alternative to those is Kdenlive, which in my opinion is just the perfect application for non-linear video editing in KDE. Kdenlive supports various video formats, includes video and audio effects, and it organises your work in projects. You can create, crop, delete and bassically manipulate videos easily in any way possible with the help of this application.
SpeedCrunch - complex calculator
Maybe not an application which can compete with a video player or image management application, a calculator is still an essential tool for any desktop. SpeedCrunch is a complex calculator application built in Qt 4 and which also features a scientific mode.
As you can see, I decided not to include applications which come by default with KDE, so you won't find applications like Kopete or Konsole here. Instead, this is a top of KDE4 applications which I consider most powerful and (probably) essential for any KDE4 user after installing a KDE-based distribution.
Noteworthy applications left out
Unfortunately I couldn't include several good programs either because they are currently in alpha state (due to porting to KDE4 libs) or they don't have a KDE4 version. Included here is Kaffeine, a Xine-based video player which is among the top KDE players, BasKet, an extremely nice notes-taking application and not only: BasKet comes with great features which allow you to build entire projects, not only take notes. KDevelop currently has a beta release of the KDE4 port too, and it is a powerful integrated development environment which allows to program KDE applications. To say nothing about KOffice, for which the first release candidate of version 2.0 was put out just last month. A powerful music editor for Linux is Rosegarden, which is a good KDE replacement for Audacity for example.