It supports third party extensions that can add additional functionalites. Here are currently available extensions

TO install this extensions
Go to Help -> Check for Updated -> Add

Name : GeoRaptor
Location : http://georaptor.sourceforge.net/install.xml
Description : Oracle EIS extensions

Name : FourthElephant Extensions
Location : http://www.fourthelephant.com/downloads/sqldeveloper.xml
Description : Oracle Error Codes

Name : esdev
Location : http://osdn.dl.sourceforge.net/sourceforge/esdev/center.xml
Description : Enhanced Excel and Save Snippets

Installation instructions

Download the software from Oracle (http://www.oracle.com/technology/software/htdocs/sqllic.html?http://download.oracle.com/otn/java/sqldeveloper/sqldeveloper-v1.0.0-no-jre.zip)

 and appropriate JDK from Sun website (http://java.sun.com/j2se/1.5.0/download.jsp)

For Windows
Install JDK 1.5.0_06 by running jdk-1_5_0_06-windows-i586-p.exe if not already installed.
unzip sqldeveloper-v1.0.0-no-jre.zip to a root installed directory.  for e.g. c:\program files. It will automatically create a subfolder with name sqldeveloper.
You must use the Use folder names option.
Double click and run the sqldeveloper.exe. It will ask for the location of java.exe. Provide the path for the JDK you installed before.
Create connections to your oracle database by doing the tnsping to the particular database service on the dos propmt and
note the hostname, port and sid and provide it during the setup.

For Linux
Install JDK 1.5.0_06 by running jdk-1_5_0_06-linux-i586.bin if not already installed.
unzip sqldeveloper-v1.0.0-no-jre.zip to a root installed directory.  for e.g. /app. It will automatically create a subfolder with name sqldeveloper.
Add following lines in the sqldeveloper/jdev/bin/ide.conf at the end

SetJavaHome $HOME/java/jdk1.5.0_06  (Set it to the directory where you have installed the jdk).
SetSkipJ2SDKCheck true

Start sqldeveloper by issuing the following command. First go to the directory sqldeveloper

cd sqldeveloper
sh sqldeveloper

or alternatively create a shell script to start this in background. You must have DISPLAY variable set to correct hostname.
for e.g.
export DISPLAY=hostname:0.0

cd $HOME/sqldeveloper
nohup sh ./sqldeveloper > $HOME/.sqldeveloper/sqld_start.txt 2>&1 &

You might get the following message. Ignore it.

Oracle SQL Developer 1.0
Copyright (c) 2005 Oracle Corporation.  All Rights Reserved.

Unable to find configuration file: jdk.conf
Working directory is /app/cportal/sqldeveloper/jdev/bin
Initializing.. oracle.dbtools.raptor.explorer.jdev.ConnectionDialogAddin@3bc1a1
Assert: SQLView initedAssert: Unknown Node:8: USER
Assert: Unknown Node:8: SHARED QUERIES
Assert: Unknown Node:8: TABLE EDITORS
Assert: Unknown Node:8: VIEWS
Assert: Unknown Node:8: MVIEWS
Assert: Unknown Node:8: SYNONYM
Assert: Unknown Node:8: SEQ
Assert: Unknown Node:8: Recycle Bin
Assert: Unknown Node:8: DB Link
Assert: Unknown Node:8: MVIEW LOG
Assert: Unknown Node:8: PLSQL
Assert: Unknown Node:8: TRigger
Assert: Unknown Node:8: INDEX

Create connections to your oracle database by doing the tnsping to the particular database service on the dos propmt and
note the hostname, port and sid and provide it during the setup.