Do I really need to say anything? Okay. Fine. I'll just copy and paste from my site...
Luigi's Adventure at the Sea
The story tells of Luigi getting stranded on Yoshi's Island due to a trick by Waluigi. While Luigi tries to fit in with the native yoshis, who don't even know what humans are, Mario has his hands full with a misadventure of his own.
Luigi the Yoshi Leader
In this sequel to LAS, Luigi returns to Yoshi's Island to relax and have fun with the yoshi herd. But only a while after his arrival, two zookeepers who are trying to save their zoo appear and attempt to capture the yoshis. Luigi, with the aid of Mario, Wario and even Waluigi and a few unexpected friends, does what he can to protect the yoshis and track down the locations of a couple herd members that never even appeared in LAS.
LAS3: Luigi's Eggsellent Adventure
In Luigi's biggest adventure yet, many of his yoshi friends have been turned into eggs and scattered across the world and it's up to Luigi to round them all up. He won't be alone of course. Along with Mario, Wario, Waluigi and the yoshis that escaped, Luigi will pick up a new ally during his quest.
The Mario Gang's Wacky Vacation
This tale...well...doesn't have a real plot. Basically, Peach gets fed up with dealing with the Mushroom Kingdom and decides to take a vacation. So she, Mario, Luigi, Wario, Waluigi, Toad, Yoshi, Bowser and D.K. (and later, Daisy and Kirby) venture out to the tiny isle of Phillip Island, which is owned my Luigi's high-school buddy, Phil. Paradise, this is not. And just about anything can happen here. Toad swearing! Luigi and Bowser are friends! Wario plays Strip Black Jack and loses on purpose! It's all here.