.htaccess
Code:
RewriteEngine on
RewriteRule ^sponsor1$ http://sponsorurl1.com/ [R=302,L]
RewriteRule ^sponsor2$ http://sponsorurl2.com/ [R=302,L]
RewriteRule ^sponsor3$ http://sponsorurl3.com/ [R=302,L]
RewriteRule ^sponsor4$ http://sponsorurl4.com/ [R=302,L]
http://site.com/sponsor1 would then redirect the surfer.
You could do 301s if you didn't want to intentionally scrape your sponsors.