This is a post that I will edit from time to time, adding more links as I find them.
1. This company has free and commercial icons: dryicons.com. I like the iconika icons (available in red, blue, green, grey – here is a sample in blue: http://dryicons.com/icon/iconika-blue-icons). One thing I really appreciated from this site, is that you get the option to download the icons at the size you need. This saved me a lot of time messing around with image resizing…
2. Fotolia.com is a good site (the only site I have personally paid so far to buy photos and vectors). Not as cheap as it used to be, but still good value, considering the amount of material that’s posted there.
3. Vector Stock sells vectors from various creators: www.vectorstock.com
I like the concept of the hand-drawn icons from this artist: Azzzya. This is a great place to pick up nice designs for presentation backgrounds and office reports.
4. Another interesting site is Wikimedia. You can use whatever you need, but you may have to search a lot to find something you need/like. Definitely worth looking into, if you have the time.