I have a schengen visa issued by the Netherlands embassy when I was travelling to Holland... And it's valid for a year.. I now want to travel to Spain and wondering if I can just fly direct .. Or do I need to apply for a new visa ?
No you don´t need to. You may be asked on the border why you travelling to Spain with Holland visa and you may easily explain that you traveled to that country earlier (showing border control stamps if necessary) and now just using your Schengen visa to visit Spain. For example, I've recently visited Germany with Schengen visa issued by Spain.
All the point of having Schengen Visa is that you can use it for all countries belonging to Schengen area/agreement. (you may also see the same list with some more details at scheschengenvisainfo.com) The only rule is that if you applied in country A for Schengen visa you supposed to visit country A first and not any other country, otherwise, you may have difficulties with getting your Schengen visa from country A next time.