Hollywood stars have plenty of reasons to celebrate this week. The Twilight Saga's Nikki Reed marked her 23rd birthday with a sinfully sweet potion at The Sugar Factory Brasserie inside the Paris Las Vegas Hotel. Zooey Deschanel charmed a crowd of TV execs when she made an appearance to promote her upcoming FOX series, The New Girl. And Jessica Alba helped welcome the arrival of German luggage designer RIMOWA to Los Angeles' Rodeo Drive. Click through the gallery to see these stars plus Gwen Stefani, Matthew Morrison, Paula Abdul and more celebrities inside last night's hottest parties!