==must be installed==

RegexPlugin
http://www.intellij.org/twiki/bin/view/Main/RegexPlugin

unitTest: better (and also bigger) than JunitGenerator

CVS bar: VCS bar is too old

SpellCheckPlugin
http://www.intellij.org/twiki/bin/view/Main/SpellCheck

DBHelper: more powerful than  SQL Query Plugin. Maybe the latter will be removed after I am familiar with DBHelper.
http://ca.geocities.com/davidhuo2003/dbhelper.html

Rearranger: use it in canonical class, such as bean  
http://www.intellij.org/twiki/bin/view/Main/RearrangerPlugin

simpleUML  
http://www.intellij.org/twiki/bin/view/Main/SimpleUML

Key promoter: Shows to user how easy he can make same action using only keyboard


==good for using==

IdeaMouseGestures: IdeaMouseGestures plugin provides easy way to assign Idea's menu items to mouse gestures.
http://www.smardec.com/products/idea.html

KonaWorks PowerPack: A new code action to easily insert an expression inside a string literal
http://www.konaworks.org/

DragNDrop: This plugin will allow a user to drag and drop files into the main pane of IntelliJ

Workspaces
http://www.intellij.org/twiki/bin/view/Main/WorkspacesPlugin

RemoteTail: The plugin allows the user to view the content of a file which could be on a different computer.

Jump to Usage: Simple plugin to make navigation to element usages easier than using Alt+F7.
http://sourceforge.net/projects/jumptousage

Library Finder: helps to find library files (.jar/.zip) for a "fully qualified" class/resource name.
http://code.google.com/p/libraryfinder/

IntelliLang: IntelliLang is a combination of three basic kinds of functionality that are meant to support the developer in dealing with certain tasks that relate to (custom) languages in IntelliJ IDEA.
http://www.jetbrains.net/confluence/display/CONTEST/IntelliLang

XPathView + XSLT-Support: required by IntelliLang
http://www.intellij.org/twiki/bin/view/Main/XPathViewPlugin

Tabifier: The tabifier plugin retabs Java code so that syntactic elements are aligned vertically. 
http://www.intellij.org/twiki/bin/view/Main/TabifierPlugin

Struts Assistant: it will conflict with Struts Plugin, so Struts Assistant is enough.
http://www.intellij.org/twiki/bin/view/Main/StrutsAssistant

IdeaSpring: Helpful to edit applicationContext.xml. But it is even a commercial one! So it has to be re-download every 15 days.

XFile: defaul Changes is good enough, so maybe it will be removed later.
http://www.echologic.com/plugins/xfiles.html

UpperLowerCapitalize: just use the capticalize, since there is default upper/lower in IDEA
http://www.intellij.org/twiki/bin/view/Main/UpperLowerCapitalize

IdeaJad: a popular decompiler
http://www.tagtraum.com/ideajad.html

SyncEdit: Remove the default shortcut "Tab" (for "Next SyncEditable Word"), since I need "Tab" work normally.

World of Java
http://www.worldofjava.org/

Scratch Pad: Additional editor tool window to hold pieces of code temporarily. No syntax colouring, intentions or error highlighting.
http://www.intellij.org/twiki/bin/view/Main/ScratchPadPlugin

HTML Preview: Provides preview for html files using Mozilla browser


==learning==

TabSwitch: Open files/tabs switcher

CVS Report for IntelliJ IDEA: CVS Report for IntelliJ IDEA is a free open source plugin for IntelliJ IDEA to generate statistical reports from your CVS repository.
http://vcsreport.sourceforge.net/

RssPlugin: RSS Plugin for viewing RSS feeds within IntelliJ.
http://www.intellij.org/twiki/bin/view/Main/RssPlugin

Hibernate Tools: better than hibero which is even a commercial one!

Smart Introduce: how to use it?
http://www.intellij.org/twiki/bin/view/Main/SmartIntroducePlugin

GroovyJ: hoping it can become more powerful
http://groovy.codehaus.org/IntelliJ+IDEA+Plugin


==not very useful, so can be ignore==

Code Outline: Have not found anything useful  (dosen't work in IDEA 6.0)

OpenContainingForder: Plugin for Viewing folders with Windows Explorer, but cannot open a folder. So "Alt+Shift+E" is better 

LineMover: default "Ctrl+Shift+Up/Down" is enough.

SQL Query Plugin: DBHelper is good enough

SQL script editor: DBHelper is good enough

JavaDoc Browser: the window can't be big enough, so acutal browser (such as fire fox) is better.

BTW: if you have any good recommendation, welcome to share with us.