Here are some of the resources we regularly use ourselves which you might find useful too.
Our favorite site for commercial fonts to match customer requirements or find something new for creative projects. Owned by Monotype the site provides over 900 professional-quality free fonts as well as commercial fonts and webfonts. Features the What-the-Font service allowing you to identify fonts you've seen in existing material.
Excellent online tool to generate vector halftone effects. Lets you upload a bitmap image and choose from a wide range of effects and settings. Then export to SVG to import into CorelDraw, Illustrator, Inkscape etc. As well as providing a creative alternative to auto-traced photos it is ideal for creative and background effects for vector illustration. Free trial version available. SVG export requires a minimal one-off payment.
Source of vector files for well known logos. Similar site to Brands of the World (see below) not as extensive but supplies in EPS format and we prefer the quality. Much better with UK logos - and they've still got their McDonalds up.
Claims to offer the largest collection of vector brand logos in the world (except any major brands like McDonalds who have blocked theirs). Found some gaps in UK-related coverage but generally extensive. Supplies in PDF format which means you're depending on the import function in your illustration program. The images appear to be auto-traced but the quality is generally good although some gradient areas were imperfect. It also means connected characters are not seperate objects.
Optimise the PNG and JPG files used in websites to improve download speed without loss of quality using this FREE web-based optimisation service.
If you've got a recommendation for something we should include here, simply send us details via the contact page. We'll check it out as soon as possible and are happy to list anything which adds value for designers and illustrators.
We think each of these genuinely adds value for any designer - if you find any issues with any of them please let us know so we can advise other designers.