In honor of Justin Bieber's 18th birthday today, we're taking a look at the top ten breakout YouTube stars! Who knows, if Justin hadn't achieved such incredible levels of fame after being discovered on YouTube, maybe the rest of these artists wouldn't have followed his lead!

Justin Bieber

It all started with a YouTube video of 13-year-old Justin Bieber singing Justin Timberlake's "Cry Me aRiver." He soon became Canada's #1 most subscribed musician and caught theattention of Usher, who immediatelysigned him to his recording label. After the debut of Justin's album My World, he became the first artistever to have seven songs from a debut album on the Billboard Top 100!

Related video: Justin Bieber & Selena Gomez Star in Hilarious Viral Vid!

Greyson Chance

After a video of 12-year-old Greyson Chance performing Lady Gaga's "Paparazzi" at hisschool's 6th grade music festival hit YouTube, Greyson was invitedby Ellen DeGeneres to perform on her show. The video is pretty amazing – not only is Greyson crazy talented, I lovewatching the girls in the background, in total AWE. After his appearance on Ellen, Greyson recorded his debut albumand went on a concert tour last summer. He also guest-starred in twoepisodes of Raising Hope!

Rebecca Black

Rebecca Black'ssong "Friday" might be auto-tuned and over-produced to death, but youhave to admit, it's pretty damn catchy. "Kickin' in the front seat,sittin' in the back seat, gotta make my mind up, which seat can I take?" Afterthe release of this video, Rebecca was catapulted to fame (infamy?) and she wasinvited on almost every major talk show. Gleeeven did a rendition of the song! "Fun, fun, fun, fun…"

Related article: Justin Bieber Involved in a Heated Video Game Lawsuit!

Amber Lee Ettinger

Her real name is AmberLee Ettinger, but you probably know her as Obama Girl. Her song "CrushOn Obama" became in an international sensation during the 2007-2008presidential campaign. With lyrics like "I cannot wait till 2008. Baby,you're the best candidate!" the video was named by Newsweek as a Top 10 Meme of the Decade and has been viewed onYouTube over 120 million times!

Alexa Goddard

Alexa Goddard isa British R&B singer who promoted herself through YouTube by singing hitsby Adele, Katy Perry, Beyoncé, andother famous artists. And it totally worked – she went on to become a professionalrecording artist and charted in the UK Top 40! Check out her rendition ofAdele's "Someone Like You" above!

Esmeé Denters

Esmeé Denters isa Dutch cover artist who performed hits by the likes of Justin Timberlake and NatashaBedingfield on YouTube. After attracting attention of Timberlake, shebecame the first artist to be signed to his record label Tennman Records and shenow has a career recording and performing her own original songs. Check out Esmeésinging Natasha Bedingfield's song "Unwritten" – with Natasha backingher up! Talk about a singer's dream come true…

Related article: Go Viral: Johnny Depp Isn't a Jerk, But He Plays One on TV

Julia Nunes

Julia Nunes isa singer, guitar player, and ukulele player who has a major following onYouTube. She sings songs of all genres, including "Accidentally in Love"by Counting Crows, "Survivor" by Destiny's Child, and "All MyLoving" by The Beatles. She's performed with Ben Folds and performs regularly at the Bonnaroo Music Festival. The girl is awesome. Watch her ukulelerendition of "Build Me Up Buttercup," and I challenge you not tosmile when her friends pop up in the background to sing backup!

Maria Aragon

Filipino-Canadian piano player and singer Maria Aragon caught the attention ofthe world when Lady Gaga posted alink to her YouTube rendition of "Born This Way" on her Twitter pageand wrote, "Can't stop crying watching this. This is why I make music. Sheis the future." Maria went on to perform on the Ellen DeGeneres Show, for Canadian Prime Minister Stephen Harper,and with Lady Gaga herself at the Monster Ball in Toronto! Not bad for a 10-year-old!

Marié Digby

Marié Digby receivedmajor attention after posting her video of her singing an acoustic version of Rihanna's song "Umbrella" onYouTube in 2007. She had a hit single with the cover, and now she's a bona fidemusic star, with three successful studio albums, including one in Japanese!

Sam Tsui

Sam Tsui'sYouTube mashup of Bruno Mars'"It Will Rain" and Adele's "Set Fire to the Rain" instantlycatapulted him to internet superstardom. He has a huge fan following now and hisvideo for his awesome cover of BritneySpears' song "Hold it Against Me" (which is widely considered tobe better than the original) was even featured on Britney's official website!

Who is your favorite YouTube breakout star?

Videos courtesy of YouTube