Jeff Jonas has an article on using war machines in Warhammer Ancient Battles. There are some nice pictures of painted Roman war machine models, and a good discussion of their use by the legions.
The Illustrated History of the Roman Empire website is just that — a neat historical site with loads of pictures and interactive map. There is lots of fodder here for historical wargamers.
Eye candy time. Here are some pictures of really well-done Early Imperial Romans. I like to have pictures like these to use as a painting guide.
The Warhammer Ancients rules book is full of point values, but it’s sometimes difficult to tell exactly how many of each troop type you should have. If you’re into Romans, here’s an example army list for you.
This free paper model of a Roman villa seems to be somewhere between 15mm and 20mm size. You could scale it on your printer, though to make it work in other sizes.