sometimes, you may find a cool icon set that is only available in SVG format. A few days ago, Forum Nokia (registration required) launched "SVG Converter 0.95 (Beta)", a tool which converts SVG files into SVG tiny or PNG format in a few clicks; the latter might be helpful especially for Nokia Series 40 theming. In the tools' export dialogue, you can select the icon size you need (16x16 px, 32x32 px, 48x48 px, 64x64 px, 96x96 px, 128x128 px or a custom size). The original SVG icon stays untouched.
As the tool is still in Beta status, there may be a few issues. When I tested it, it crashed a few times occasionally but all in all converted the majority of my test icons without problems.
Download SVG Converter 0.9.5 (Beta)
Download the accordant User Guide (PDF)