Esperanto
The Esperanto is a two-door car available in all of the GTA III era games and in Grand Theft Auto IV and can be found fairly widely throughout the map in all the games in which it appears.
GTA III Era
The Esperanto is based off the 1975 Cadillac Eldorado, hence the name.
Although performance aspects vary slightly between games for this vehicle, acceleration and speed are generally very satisfying with the rear-drive V8 powertrain. The one common problem with the Esperanto, however, is its enormous bulk, which means that once at top speed, the vehicle can be a real challenge to bring to a quick stop. This becomes a problem in GTA III, where Claude is unable to jump from a moving vehicle, exposing him to serious danger in a fast-moving Esperanto about to explode. The car's handling is highly responsive, but again, the vehicle's size and weight cause problems, predisposing it to 2-wheeling and loss of control when overcompensating for a swerve at high speeds. However, its huge mass absorbs damage quite well, making it an excellent vehicle for ramming smaller ones (and even comparably sized ones) off the road.
GTA IV
The Esperanto in GTA IV is very different than in previous iterations of the series. Rather than being a large 2-door luxury vehicle, it's now a mid-sized 4-door sedan. Its styling inspiration appears to be a pastiche of common late-70s/early-80s American car styling rather than a clone of any one car. The front end and sides are similar to a 1977 Impala though the headlights and signal indicators sit more flush with the grille and front trim, much like a 1982-'85 Chevy Celebrity, while the rear-end features bustle-back styling found on the Cadillac Seville, with taillamps lifted from a 77-81 Pontiac Bonneville, and the chrome on the bottom of the sides looks reminiscent of a Chevy Caprice's from around the same era.
A modified variant (with official taxi ID plaques) is driven by Niko Bellic's cousin, Roman and is simply called Roman's Taxi when getting into that particular Esperanto. When free cab rides from Roman's garage are unlocked, an Esperanto will be sent to pick Niko up.
The Esperanto was one of the first cars to appear in any media in the run-up to GTA IV's release. It was often mistakenly referred to as an "Albany" due to the fact that the word is conspicuously emblazoned on the grille. In actuality, Albany is the "make" of the car (other vehicles are seen with the Albany nameplate on their decklids and logos, such as the Primo).
Trivia
The reason why the Esperanto looks different from from the GTA San Andreas model is because a newer model features in GTA IV.
One of the first cars to appear in any media, this black saloon is thought to perhaps be a minicab belonging to Roman, Niko's cousin. In its appearance in the third trailer it is shown leaving the building rumoured to be Roman's taxi rank with both Niko and Roman himself inside. A minicab/taxi licence number sticker can clearly be seen printed on the rear.