Alyson Hannigan, Kenan Thompson, More Stars Honor Michelle Trachtenberg

Michelle Trachtenberg is being remembered for the positive energy she brought to set by many of the stars she worked with over the years ... after her death was announced today.

Numerous stars have taken to social media to show their love for the late actress, sharing stories from their times working with her.

Kenan Thompson posted a pic of the two from way back in the day ... calling her the first Nickelodeon movie star -- as well as one of his close friends.

Alyson Hannigan -- who appeared alongside Trachtenberg in numerous episodes of "Buffy the Vampire Slayer" -- shared a picture of the two together and said she exuded loving energy at work.

Others added their own tributes ... including Kim Cattrall, Snooki, Lele Pons, Matt King, Shane Dawson, Iris Apatow, Shawn Ashmore, Brandi Glanville and many more bold-faced names.

A big takeaway from a large swath of the tributes online ... many are thanking Michelle for being an integral part of their childhoods.

Michelle was found unresponsive in her New York City apartment Wednesday ... and, first responders pronounced her dead at the scene. A source told us Michelle had a liver transplant in the past year -- unclear if that played a factor in her tragic death.

She was 39.