The Queen's Gambit has been around for at least 500 years. There are a plethora of variations attached, that in the main, fall into two distinct categories. They are QG Accepted and QG Declined.
For a long time this particular system was not especially common in top-level chess as the closed openings did not lend themselves to explosive attacking games right from the off.
When Steinitz and later Tarrasch developed the concept of positional play it underwent a renaissance and, due to it's endgame possibilities, became extremely popular. It has receded since it's zenith in the 20s and 30s but is still used by masters from time to time.
The Smith Morra Gambit is named after Frenchman Pierre Morra and American Kenneth Smith, both of whom studied it thoroughly. It is used by White to counter the Sicilian Defense by Black.
People are split on who has the better line, White on the Smith Morra or Black on the Sicilian. One thing for sure is that there is plenty of theory to cover on the Smith Morra as there are plenty of roads this one can take you down.
The Smith Morra is well worth a look if you're a 1.e4 player and you're sick or those smug Sicilian fans! It's probably as successful as 2.Nf3 at club level.
The Vienna Gambit is so named because it was first played in the mid-nineteenth century by a Swiss guy called Carl Hamppe. The thing was though he worked as a government official in Vienna, Austria and used to play a lot of his games there.
This is essentially a delayed King's Gambit with the Queen's Knight developed to c3 on the second move and the f-pawn offered as a sacrifice with 3.f4.
This is a gambit of it's time, aggressive as was the style at that time. The idea behind this is to give White the chance to pressurize Black's soft center down the bishop's file at f7. It is generally believed to lead to equality with best play from both sides.
The Reti Gambit was introduced by Richard Reti in the 1920s. It is designed to counter the French Defense. White plays the King's pawn up to tempt the Black Queen's pawn to capture it, effectively ripping the head off the French Defense.
This one will take you down some novel positions and may disorientate your opponent. Reti himself stunned the mighty Capablanca with his hypermodern inventions. So if his ideas could throw someone like Capa what chance for your friends down the club!
There are several possibilities for the Reti and there is a lot of theory attached. But if you do your homework and get familiar with the main lines, you will have a real gem, especially below master level.