Hmm, here's some of what I have. Not necessarily that recent, but generally popular... But I'm from Canada, so maybe we don't get exactly the same songs as people from the US.
But the most efficient stuff i've found remains japanese songs. But not everyone likes those.
In the following lists, if you're like "wtf" because a song is sung by a man, keep in mind that it's useful is when you try to make it sound feminine.
Kelly Clarkson - Behind These Hazel Eyes
Fergie - Big Girls Don't Cry
About anything from Mika (challenge 1 : reach his notes / challenge 2 : makes it sound feminine)
Ke$sha - Tik Tok
Stuff from Green Day or Linkin Park
Hilary Duff - Coming Clean
Katy Perry - Teenage Dream ; Hot 'N Cold ; I Kissed a Girl ; many more
Natasha Bedingfield - Unwritten
Stuff from Spice Girls
Stuff from Rihanna
Flipsyde - When It Was Good
Oh, let me add some Japanese stuff incase you want to try. They're all useful, but some of them are so hard I still don't sound very feminine with them even after months, so feel too down.
AAA - Q ; Mirage ; Red Soul
Azu - For You
Bump Of Chicken - Karma
Buono! - About anything
Death Note - The World
FLOW - Re:member
Guardians 4 - Anything
Hearts Grow - Kasanaru Kage ; YURA YURA
Ikimono-gakari - About anything
Inoue Joe - CLOSER
Lia - Tori no Uta
miwa - chAngE
Nishino Kana - Style. ; Stamp, September 1st
Ore Ska Band - Tsumasaki
Prague - Light Infection
SCANDAL - Shoujo S
Shugo Chara Egg! - About anything
Supercell - Utakata Hanabi
Tainaka Sachi - Wasurekakeita no Kana
Tales of Symphonia - Starry Heavens
Tommy heavenly6 - Pray ; PAPERMOON ; Lost my pieces
Tsubasa Chronicle - About any sung song
Tsuji Shion - Sky Chord ~Otona ni Naru Kimi he~
Uverworld - Chance ; D-Tecnolife ; Just Melody
Yui - Life ; Rolling Star
Yuki Suzuki - Aka no Kakera
But despite all of these recommendations, your best choice is to choose a signer you like and sing her songs to your heart's content.