The only one I know of is Thunderhawk... though it's not free. At least you can try it out for 30 days. I've been using it for the past 2 weeks and I can't imagine browsing without it anymore. I'm now damned to subscribe to their service (5$/month) !
Not only does it display Web pages in portrait mode, it optimizes them for easy browsing on your PPC (the sites are optimized through their servers, hence the service fees). You can browse any site in 640x480 or even 800x600 and it's still readable. No more scrolling.
Here's the address if you want to give it a try :
www.bitstream.com
Hope this helps!