Adele had her first top 40 hit in 2009, but only became a superstar two years later.
30 Seconds To Mars formed in 1998, but released their first album in 2002 and gained mainstream success with their second release in 2005, specifically the single "The Kill" and its accompanying music video.
Coheed and Cambria - Each album from this pop-punk-influenced Progressive Metal band follows a single concept, adapted into a comic series called "The Armory Wars". They released their debut in 2002 and continue to go strong today.
Cobra Starship — A punk-pop band formed in 2006 that contributed (fittingly) to the Snakes on a Plane soundtrack, but didn't have their first major hit ("Good Girls Go Bad") until 2009.
Coldplay — Arguably the most successful rock band of the Became household names after their song "Clocks" won the "Record of the Year" Grammy.
The Darkness — A British band who helped spark a glam-rock revival in the middle of the decade. While they were quite big at home, they are best known in the States almost exclusively for their 2004 falsetto-ridden hit "I Believe in a Thing Called Love."
Nelly Furtado — Canadian singer who had a pair of hits in 2001 before fading into obscurity, but managed to make a comeback in 2006.
Gackt - Active since 1995 as vocalist of the Visual Kei band Malice Mizer, left in 1999 to pursue a solo career, eventually gaining popularity in the early 2000s and beyond. Very popular for his active participation in various J-culture scenes.
Lady Gaga — Unquestionably the biggest new star of 2009, quickly becoming the most popular star of the early 2010s before she started to fade away.
Kendrick Lamar — One of rap's most promising young artists, and one of the very few mainstream rappers respected by hip hop fans. Although he started his career in the 2000s, he never got a mainstream audience until 2012.
Maroon5 — A funk-rock band whose surprise pop success led to them adapting an increasingly commercial sound. Frontman Adam Levine gained further recognition the next decade with The Voice and his group re-establishing itself as a core pop act.
Amy Winehouse — R&B singer who had a big international hit with "Rehab" and a huge Grammy sweep the following year, before her actual personal problem overshadowed her career. Her death in 2011 reversed her reputation overnight and internationally turned the relatively obscure singer into a legend.