Rentglass.com rents Canon EOS lenses online and ships them to any of the 48 continental U.S. states (Alaska and Hawaii will be added in mid-2006). A notice on the front page of the company web site says that Nikon lenses will become available in "spring 2006".
With certain important caveats, the company looks like a worthwhile resource to bookmark, especially for U.S. photographers who don't have a nearby pro rental house. They carry a reasonable selection of Canon's wide to mid-tele standard and "L" lenses, both prime and zoom, and certain special purpose optics such as the Tilt-Shift 24mm f/3.5L.
They do not, however, offer the fast telephoto lenses that are most often rented by pros. (A company FAQ says that shipping and insurance costs on the super-teles would drive their rental prices up too high.) The most exotic teles they carry are the 300mm f/4L and the 400mm f/5.6L.
It also appears from cursory inspection that, at least at present, many of the more desirable lenses are frequently out of stock.
That said, the company rents lenses for periods of one to three weeks at prices, including 2-way shipping, that appear to be modestly lower than at least some, but probably most, big-city rental houses for most lenses. You arrange the rental online; transit days are not counted against your rental period; and the optics are shipped in a re-usable box with a prepaid return shipping label included.