Hands down my favorite: http://www.phillipmartin.info/clipart/homepage.htm

Microsoft Office Clipart Gallery online http://office.microsoft.com/en-us/clipart/default.aspx

Discovery Education Clipart http://school.discoveryeducation.com/clipart/?pID=clipart

Dorling Kindersley (DK) Clipart http://www.dorlingkindersley-uk.co.uk/static/cs/uk/11/clipart/home.html

Free Teacher Clipart http://www.teacherfiles.com/clip_art.htm

Primary School Science Clipart http://www.primaryschoolscience.com/clipart.php

Free borders, frames and scrapbook dividers http://www.leehansen.com/clipart/Borders/index.html

Clipart ETC - Free clipart for students & teachers http://etc.usf.edu/clipart/