The GIMP is a free and open source image manipulation program (it is the free alternative to Adobe Photoshop).
Open your image with The GIMP (File->Open). Then choose "Scale image..." from the Image menu:
The default settings keep the image's aspect (the width/height ratio), but if you want to distort the image, you can click on the little "chain" next to the width and height. Choose the "Cubic" interpolation for better results (unless you're working on a big image and you're really in a hurry).
Click the "Scale" button, save the image (File->Save) and you're done!