Here are some links to frequently asked questions about hosting multiple websites or sharepoint sites on an SBS server, and hosting multiple mail domains (with thanks toi the M&M ladies, Sean and Vlad)
How to publish websites with ISA 2004:
http://www.smallbizserver.net/tabid/266/ArticleType/ArticleView/ArticleID/134/Default.aspx
Hosting a website on your SBS 2000 & 2003:
http://www.smallbizserver.net/tabid/266/ArticleType/ArticleView/ArticleID/89/Default.aspx
Publishing multiple sharepoint web sites:
http://www.vladville.com/2005/08/sbs-web-sharepoint-hosting-guide.html
Hosting multiple domains on an SBS Server:
Part 1: http://seanda.blogspot.com/2004/10/hosting-multiple-domains-on-sbs-2003.html
Part 2: http://seanda.blogspot.com/2004/10/hosting-multiple-domains-on-sbs-2003_22.html
Part 3: http://seanda.blogspot.com/2004/10/hosting-multiple-domains-on-sbs-2003_25.html