The Buick is one of the longest running easter eggs in the Star Wars: Rogue Squadron series. Based off of one of the employee's cars at Factor 5, the series developer, it has been featured as both a ground and land vehicle piloted by both B-1 Battledroids and Rebel Alliance pilots. It's license plate bears reference to the car's real life owner as well.
As odd as it may seem, Santa Claus (right at the bottom of the image) was chosen as an Easter Egg in the original X-Wing game. He appears in the main lobby so long as your computer is set to December 25th. This wouldn't be his last time appearing in a Star Wars game though...
Santa Claus' second appearance would come in LEGO Star Wars II: The Original Saga. Entering two different codes ('CL4U5H' and 'TYH319') would get you Disguise 3 parts for character customization. Using these parts would enable the player to turn their character into LEGO Santa Claus, and use him to to battle either the light or dark sides of the Force.
Hidden exclusively in the Star Wars: The Force Unleashed's demo, above a platform accessed only by a series of crates precariously stacked, is the helmet of none other than Boss, the protagonist from Star Wars: Republic Commando.
Curiously, the Easter Egg was removed from the final game, possibly hinting an internal wish to see a return of Delta Squad.
On the flipside, it may be possible the presence of the helmet is a reference to this similar Easter Egg hidden in Republic Commando itself. A lightsaber with a burned body of a clone are all that remain in the shaft of an under-siege Republic cruiser.
Upon finding the lightsaber, Boss will stoically comment "A weapon for a more civilized age. Well guess what, times have changed." It was both a great little wink to the original trilogy and a tone setting moment during a level where the majority of your squad has been captured, leaving you outnumbered and isolated in the dead cruiser.
Like many of the games developed around the time of Star Wars: Episode 1, Star Wars: Starfighter had a number of droid gags. However, two were incredibly hard to find, this casual bacon-fancying droid certainly counting as one of those two.
And here is the second hard to find Easter Egg in Star Wars: Starfighter: the Cowboy Droid. Featured in a single mission aboard bonus objective escape pods, most players missed him before their lasers blew him to hell.
Star Wars: Galactic Battlegrounds has a lot of Easter Eggs to begin with (ranging from remaking scenes of the original trilogy to Expanded Universe references and dev notes written in the official Star Wars alphabet), but one of the best is a hidden bonus mission in the first mission of the second Wookiee campaign. It could only be accessed by having all the Wookiee workers collect carbon and repeatedly switch roles to drop the carbon.
While there was no Naboo faction campaign, they appear in a mock-mini mission where you rescue Naboo's queen from Tusken Raiders and Darth Maul. The mission's objective was simply to save her at any cost, so any of the other cast members appearing (including young Anakin Skywalker) could be left to die at the hands of your enemies. Completing the mission gave you a success even if you never were able to complete the main mission.
It's impossible to list them all the various gags and elements hidden in the game, but the sheer number of them is astounding, having more than any other Star Wars game.
Technically Star Wars: Dark Forces II: Jedi Knight has three different references to Max from Sam & Max. Lucas Arts had the rights to at the time thanks to working on Sam & Max adventure games, so not only did they hide a texture of Max's face pasted across a wall (similarly hidden in Star Wars: Shadows of the Empire), but they included a "Max Vision" and Max as an NPC in one of the game's levels.
Max Vision drew strange green lines across all the game's elements, while the NPC could either be befriended by freeing him and his allies from a guard, or made an enemy by attacking him. Max wields the same gun as series protagonist Kyle Katarn, and featured his own custom assets.
In another great homage to Lucas Arts' early adventure game days, Star Wars: The Force Unleashed II includes two references to Guybrush Threepwood, protagonist of Monkey Island adventure game series.
He is both a playable skin (named Guybrush Threepkiller like Force Unleashed protagonist Starkiller) in the game and shown in statue form on Cato Nemodia near another Easter Egg hologram advertising Jabba's Palace. Guybrush's statue points towards the exit of the room he is in, although sadly is one of the non-interactive parts of the environment.