The Movie Blog posted a hysterical article about actors who fooled you into thinking they could act. This is SUCH a good list, and I agree with everyone on the list, including some of the suggestions in the comments, like Cuba Gooding Jr., Jennifer Lopez, John Travolta, Katie Holmes, and Nicolas Cage (who I consider to be the worst actor EVER; it's worse when they're dramatic actors who take their roles seriously and still suck).
I would definitely add Lindsey Lohan to the bunch. She was great in The Parent Trap, but she was a kid. People need to stop hiring her so she will go away! And she can take her craptastic leggings with her.
P.S. I seriously dislike Cage. What did he ever give us but pouty faces and a mouth that barely moves? City of Angels (such a horrible remake). National Treasure (I pretend like this movie and its sequel were never made).