Ubuntu is a good OS which is free of cost.
You can do all your developer works on this. You can find more about ubuntu in ubuntu.com.
Here i am going to give a simple example why ubuntu is better:
My brand new HP laptop comes with windows vista home premium version. Which really sucks because its too much graphics, for everything it pops up with a dailog, you need to say continue. I also had 2007 office which is only free for 3 months. Later i have to buy a new office 2003 or 2007 for around 7,000 rupees !!!!
I am using a ubuntu 8.10 currently. Its free, free and free. No need to pay to anyone. This comes with Open source office tool called Open Office 2.4. In windows if you have office 2003 then you can’t open the office 2007 files. They have very different format and extension is also different. But ubuntu’s Open office can open any document of office 2007 or 2003.
Thats the speciality of ubuntu.