There is a new option at Google, Google Webmaster Tools. When you register for an account and upload your site to be reviewed here, there is an option to participate in "enhanced image search". It seems to be a good idea for anyone having unique images on their website.
Open Directory's Category for Medical and Scientific Illustration (I am a DMOZ editor. See my DMOZ profile & bookmarks)
Portfolios.com
Sam's Directory
Sbe86.org
Interested in listing your site and/or your artwork at sites that require a membership fee? See
Listings Requiring a Membership Fee
For more listings, see Yahoo's List of Virtual Galleries
Also note that many of the artists' associations will link to your website from theirs if you are a member of their organization. See a list of artist's organizations here.
Finally, don't forget about your college. Often they will link to alumni web sites. It's worth checking into.