Who are some Disney Channel and Nickelodeon stars that went on to become major actors?

Lindsay Lohan... the best Disney success story there is (Mickey Mouse should start his own rehab). Kenan Thompson from Kenan and Kel is on SNL. Amanda Bynes just came out of retirement (as if she had much to retire from). Megan Fox actually starred in Confessions of a Teenage Drama Queen with LiLo so maybe you can count her even though her acting is bleh. Zac Efron is another one but these people aren't really all that major.