There are various websites available on the web which are providing clip art for free. They can be used for commercial or non-commercial use. You can scale them to any size based on your requirement in your projects. Those websites are:-
This website provides you lots of clip art or graphics. You just have to open the website on your browser and to select the clip art which best suites you according to your requirement. After the selection just click on the clip art and here you'll find the date when the clip art was created, its file size, description and different download options like big image,medium image,small image, pdf, wmf etc. You can also edit the clip art here.
This website has arranged all the clip arts under various groups like architecture, industrial, miscellaneous, science, hardware and tools etc. You have to just click on the category related to your project. If the groups are further categorized then go for the selection and you'll get the required clip arts.
Clker.com also provide some of the license free clip arts. You can draw and upload your clip arts too to this website but you have have to login or register to upload picture to this website.
Dreamstime provide royalty free clip arts to us and illustration. Millions of stock images and illustration are there on this page and also thousands of new images are added daily.
The above mentioned websites contains clip-arts free of license and hence you can use them for any type of use.