My favorite starship has to be the refitted USS Enterprise, first seen in Star Trek: The Motion Picture. The design of the original ship went through a major upgrade and giving the ship a more realistic design, something that looked like it could be up in orbit right now, and this still holds true over 30 years later.
This gave rise to other starship designs, most notably, the USS Reliant – the first time we saw a different class of starship in Star Trek – that was introduced to us in the second movie, The Wrath Of Khan.
Since then, and all through the 2280 – 2300 the designs of the ship continued to follow the same principles, using the same components for different ships. Of course, this was a cost saving measure employed by the production crew to save money, using pieces of the AMT/ERTL models of the USS Enterprise and adding custom parts.
Some have said this cheapened the designs of the ships, that it was lazy and unimaginative, but consider the fact that Starfleet would have designed and built other ships using surplus parts.
Design varients of the Constitution-Class include the afore mentioned USS Reliant, a Miranda-Class, the USS Stargazer, a Constellation-Class and even the USS Excelsior, which had different components but essentially looked very much like the Enterprise. The USS Saratoga and the USS Brattain, both Miranda-Class ships were seen much later in but were built around the same time. (The Saratoga first appeared in Star Trek IV: The Voyage Home).
These ships basically had the same saucer section, the same warp nacelles and the same bridge modules, giving a uniformed look across the whole fleet. This of course happens in real world naval vessels, where identicle component parts are used in different ship classes.
Below are some images of these ships, with a few fan-made varients that follow this design standard.