Some people are what is labeled a power user. I am one of these people. No matter how fast I get my system, or how quick of programs I have, it is never good enough. There is always at least one program that I could swap out for a more advanced, text-based counterpart that increases performance just a bit. Luckily, you don't have to use text-based programs without graphical user interfaces to get blazing fast speeds on Linux. There are tons of open source alternatives to the mainstream programs that cost loads of money.
I use these programs soley because I am a resource-hungry fiend. However, even to the normal user, you will see an enormous difference in execution and task speed when comparing these programs to their more graphical oriented counterparts.
This Null Byte is a bit of a change of pace. I'm going to introduce you to many open source, lightweight applications, and the categories that they belong to (i.e. text editors, browsers, etc). These are all of the best lightweight programs available to us, in my opinion (for what it's worth). This should answer all of your questions when it comes to necessary programs. These are the best of the best.
Categories with only one item means the program is that good. All of these programs are done by package name, so they can be installed by calling them what they are named here.
These control the desktop environment or window manager selection and log in.
Tiling window managers are extremely light graphical interfaces that can tile your windows in an easy-to-access fashion.
Floating managers are about the same as tiling. Except they do not tile. Also very light and quick to respond.
Desktop environments are full fledged graphical environments. When you hear DE, think of a Windows-like interface.
Shell interpreters. This is what is used to execute our terminal commands. The default Linux shell is bash.
These are to extract or archive files (.zip, .rar. etc.).
File managers are what you use to navigate directories with a graphical interface and manage files. Point, click and drag to manage files on your system.
GIMP is an advanced photo editor that some revere as even better than Adobe Photoshop.
Image viewers are the small, non-editing programs used to look at picture files. This is what you would see after you double-click a picture file.
Instant messenger clients are for instant messaging protocols. Pidgin supports nearly every protocol in existence and has plugin support (as well as great plugins already).
IRC clients are used to connect to IRC servers and channels.
Network manager are what is used to connect to your internet connection.
A word processor is just an advanced text editor with formatiing options. Think MS Word.
A package manager manages what software is installed on your system.
Zathura is a lightweight, terminal-like PDF viewer. Incredibly small and fast. It also saves bookmarks.
RSS readers are used to feed you the latest updates from all of your RSS subscriptions.
Audio players are used to play music files.
Tint2 is a fast system panel with a task tray that works across all platforms. It is highly customizable and fast.
Terminal emulators are used to access the command shell from within a GUI.
Text editor are simple programs used to modify unformatted text. Usually used for notes, or for programming.
Torrent clients are used to download bit torrent files.
These play every kind of video format you could imagine. Streaming, TV, and lots of other option are available. These are better than any media player for other platforms in my opinion.
Web browsers are used to access the world wide web. Luakit is a micro browser framework that is barebones. Everything about the browser has to be customized.
The absolute best of the resource friendly programs for Linux!