The USS Monitor and Confederate CSS Virginia (often - as on Pyro's box art - incorrectly referred to as Merrimack) are two historically significant warships mentioned in virtually every book covering the history
of naval warfare. They were the first seagoing ironclads to fight each other in combat (in an inconclusive battle which inflicted little serious damage on either ship, though the Virginia was driven off
and the captain of the Monitor injured) and the Monitor itself was a revolutionary design, the first warship with a rotating armoured gun turret. Ironclad warships were employed in significant numbers by both sides - particularly
in riverine warfare - but the battle of the Monitor and Virginia is the one that has gone down in history.
USS Monitor, designed by the Swedish John Ericsson, had an extremely unconventional appearance, with minimal freeboard and a deck almost featureless other than the cylindrical turret housing two 11" Dahlgren smoothbore guns. Both hull and turret were armoured, and the turret rotation was powered by an auxiliary steam engine. Designed for coastal operations, it had limited seaworthiness and sank off Cape Hatteras in late 1862 whilst under tow. Monitor's gun turret was raised in 2002 and is now on display at the Mariner's Museum in Newport News, Virginia, along with a full-size replica of the ship and many other artifacts (including the propellor) recovered from the wreck.
Monitor gave its name to an entire class of warships - which, via the "turret ships" of the 1870s such as HMS Cerberus, arguably evolved into the seagoing armoured battleship. The "monitors" of World War I and II were rather different craft, being designed for shore bombardment, not coastal defence.
Though USS Monitor was a unique design with no sister ships, the Union Navy built many more monitor-type ironclads during the Civil War, including the twin-turreted Monadnock-class vessels.
CSS Virginia was very different in construction and appearance. Rebuilt from the wreck of the Union frigate USS Merrimack, it had a sloping armoured casemate housing a battery of guns (several calibres were carried, both rifled and Dahlgren smoothbore types) in broadside, bow and stern positions. Virginia also carried a ram, with which it sank the frigate USS Cumberland during its attack on the Union fleet at Hampton Roads. Virginia never saw action again after its famous battle, and was burned only two months later to prevent its capture by the advancing Union troops.
Both Lindberg and Pyro produced kits of these ships in the 1960s. Confusingly, although Lindberg acquired many of Pyro's moulds after their demise, their Monitor and Virginia kits are not the same. Lindberg's kits are slightly smaller and are generally less detailed and accurate. Both companies sold their Monitor and Virginia kits together as a boxed set. The Lindberg kit also included a moulded sea base to display the models as a diorama.
Lindberg's set is the most readily available nowadays - I don't think it's currently (2009) in production, but it was certainly available at retail in the 1990s. The Pyro kits are significantly better so are worth looking out for if you can find them at a reasonable price.
Although sold as a boxed set, the two ships in Pyro's set are not to the same scale; both are roughly the same hull length, so the 172'/52m Monitor scales out at 1/210 and the 275'/84m Virginia is 1/297 - which poses problems for building a diorama with the two models! Other than this, the kits aren't bad. There are some issues (such as the lack of a turret roof grating on the Monitor kit and the lack of gunport covers on Virginia) but nothing that can't be fixed with some basic scratchbuilding work, and some areas - such as the armour plating and rivet detail - are quite nice. The Monitor kit is a fairly accurate representation of the ship's appearance at the Battle of Hampton Roads (it lacks the later additions such as the sloping pilothouse armour and turret awning stanchions); the exact appearance of Virginia is still not certain, so who knows how accurate or otherwise Pyro's kit is; it certainly looks close to most drawings + paintings of the ship.
These two are the only Civil War ironclads available in plastic kit form (USS Monitor is also available as a 1/144 kit produced the French manufacturer "Battleaxe") but a number of resin kits are available in 1/96, 1/192 and 1/200 scale, as well as resin and metal models in 1/600 and other smaller scales, mostly intended for wargaming. Verlinden produce a sizeable range of 1/200 ironclads which are quite reasonably priced (though not so easy to find outside the USA)