There can be many a times when you cannot afford to spend those valuable $$$ to prove your concept or express yourself or give you the start you are looking for 
Then where do you go?? Now that open source has become such a big thing, that I don't think I need to explicitly answer this question!! But then, identifying what application to use and where can get a bit daunting (As daunting as atleast 3 rounds of googling 
)!! Anyways, here I would like to recommend 5 open source application which can get you rolling with your concept.... Note that, each of these projects are mature enough that they can be used for every mission critical operations and you can trust them to run your whole business.
- Netbeans is what you should be asking for if you are looking at creating any Rich Client Application quickly! Be it Database driven applications or a stand alone tiny application, Netbeans is the best choice for you! Anytime you wish to exploit the power of Java Technology then Netbeans is your man Not to forget, if you are planning to create a nifty mobile application which you can use to adore your cell phone then look nowhere else. Many other things like Ruby, C and C++ etc. runs smoothly!
- Productivity suites can be really expensive. But OpenOffice.org is an amazing Office application which is very feature packed and complete. And the best thing is that it comes sans price tag! Direct PDF convertion, native support to the ODF and MS Office formats and hundreds of other features very much make OpenOffice.org a-must application!
- Sun xVM (VirtualBox) would allow you to run anything on anything! Run OpenSolaris on Windows, Windows on OpenSolaris, Windows on Linux and Linux on Windows and many such other combinations! VirtualBox OSE would allow you to test your applications on whatever platform you want. With a download less than 25 MB, you are ready to go to install any platform in your system without actually needing to buy any separate hardware for each platform.
- Now that you have a IDE and Office plus a virtualization application in place, you would need something to run your Website. Drupal is a right choice if you are looking for a contect driven website. Written using PHP and supports both MySQL and PostgreSQL, Drupal can me made as feature rich as possible, using thousands of community contributed module!
- XAMPP would allow you to install the comple Apache, MySQL and PHP part of the xAMPP platform. Its a simple to use and easy to download!
The beauty of all these applications are that they run on almost all popular platforms! You are free to use which ever operating system you want to run these applications. They help you get off the ground easily. Infact, if you are smart enough, you would take the extra effort to get yourself a copy of OpenSolaris, which would cost you nothing and provide you a reliable, feature full and stable development environment. And if you are considering to do a next facebook or something then check out OpenSolaris Web Stack which would give you a SAMP platform on a platter! 
These are some of the applications I like a lot in the OpenSource ecosystem. There are million others and almost any of your needs can be fulfilled in the OpenSource world!
Now tell me, doesn't open source deserve the title of biggest enabler of our century???
