Britney Spears

Britney has always had two websites: Britney.com (managed by Jive Records to promote her music) and Britneyspears.com (her official artist website, which advertises other stuff as well as her music, like movies and fragrances).
Britneyspears.com hasn't been updated in a really long time, but it is meant to be being relaunched at some stage. That is the website that Britney has always used to post messages directly to her fans, so I can't imagine it's going to close down. It might be relaunched when her next album comes out and she starts getting more involved with promotion again.
Britney.com will keep running as well, as it is used by Jive to push record sales.
Hope that helps!
She also has an official UK site at www.gimmemorebritney.co.uk. (Run by SONY BMG). These are the only official sites I'm aware of.