Leifi was prank calls made in the middle of the night to people that had inserted classifieds in Gula tidningen (The Yellow paper).

Leifi talked with the accent of a finnish romani and was very aggressive.

He often started the calls almost polite, introduced himself as Leifi Plomeros and explained that he has seen this classified but eventually he often tried to buy weapon, drugs or sex from the person he called and got really mean when they tried to explain that they could not offer him these things. Sometimes he even tells the victims that he's going to visit them or kill them.

• Origin
  Leifi started to spread on places like Napster, single serving Lefi sites and IRC around 2000. There are threads on Flashback from around 2001 with resarch of who the prank caller is, but no certain answers is given.
  Some of these sites got over 9000 hits.
  http://web.archive.org/web//mustalainen.50megs.com
  http://web.archive.org/web//www16.brinkster.com/1leifi/index.html
  http://web.archive.org/web/*/http://www.loranga.net/leifi/

Nowadays you can listen to them at youtube, https://www.youtube.com/watch?v=WrZI9kTqPAc

• Leifi Karaoke
  Someone made an application called Leifi Karaoke. Where you can push buttons to get samples from the prank calls. A lot of people started to make their own prank calls using the application. These calls were reported to the police in the end of the year 2000.

• Fan art and compilations
  There are a two mp3 compilations with album covers

• Mainstream media
  The swedish tabloid Aftonbladet writes about Leifi and Leifi karaoke published 2001-03-14
  In 2009 IDG in Sweden featured one of the nicer Leifi calls on their list of top 100 IT-related clips, where Leifi tries to trick a kid to sell his fathers computer really cheap

• Is it a meme?
  I'm not sure how to classify it.
  It all started small on the Inernet but has grown.
  The Leifi Karaoke made people risk penalties to do their own calls and some of these calls lives on the Internet.
  His way of speaking dirty and aggressive with finnish accent is spread even to IRL. If you talk that way most of my friends at least understands that you are "doing a Leifi"

For sure it's an important part of the Swedish internet culture

[More info and research to come maybe even a translated call. And a touch up on the language if it's not sent straight to Deadpool.]