Force.com IDE Installation

Install as Standalone Application

For users developing exclusively on the Force.com platform, these installers will download Eclipse code from a public mirror and configure the Force.com IDE to launch as a standalone application:

• Windows Installer (32-bit)
• Windows Installer (64-bit)
• Mac OS X Installer
• Linux Installer (32-bit)
• Linux Installer (64-bit)

Note: you may need to install the Java JRE on your system.

Install as Eclipse Plugin

If you are already an Eclipse user, these instructions show you how to install the Force.com IDE into your existing Eclipse distribution or upgrade from a previous version:

• Force.com IDE Installation for Eclipse 3.6
• Force.com IDE Installation for Eclipse 3.5
• Force.com IDE Installation for Eclipse 3.4
• Updating the Force.com IDE from a previous version

Source : http://wiki.developerforce.com/page/Force.com_IDE_Installation

Force.com Eclipse plugin installation problem

Installation fails with error 'Md5 hash not as expected'
original error message :

An error occurred while collecting items to be installed session context was:(profile=SDKProfile, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=). Problems downloading artifact: osgi.bundle,com.salesforce.ide.api,23.0.2.201201091635. MD5 hash is not as expected. Expected: 97a6329f82c422a61e9b1bc28be7cace and found ef8b1c2b63c7d04acaa6bf41f4b8570c.

this happens due to corrupted download of jars by eclipse.To fix the problem follow these steps:
1. download all jars manually from
    http://www.adnsandbox.com/tools/ide/install/features/ and
    http://www.adnsandbox.com/tools/ide/install/plugins/
   you can use firefox plugin 'download them all' to download all links in one click.
2. put them in /features/ and /plugins/ directory of eclipse
3.run plugin installation again as you would do normally.It won't download the jars again and it will work without error.

Original post:
http://stackoverflow.com/a/9823562/1219463