Hey, It's That Guy!: From the current cast, Craig Charles of Red Dwarf fame plays Lloyd Mullaney; Keith Duffy from Boyzone plays Kiaran McCarthy; Alison King (Carla Conner) used to star in Dream Team on Sky One; Ben Price (Nick Tilsley) used to have roles in Footballer's Wives & Casualty; Kym Marsh from Hear'Say plays Michelle Conner; Alex Bain (Simon Barlow) used to be in adverts for Rice Krispies; and Chris Fountain took over the role of Tommy Duckworth in early 2011.
Plus, there's the list of people who have made brief guest appearances, which includes Sir Ian McKellen, Peter Kay & Prince Charles.
Cherylee Houston, who plays Izzy Armstrong, previously appeared on a couple of episodes of Little Britain.
Post Script Season: Probably the ultimate example — it has essentially been on a series of postscript seasons since 1960. It was originally intended to be thirteen episodes long, with Coronation Street bulldozed in a Made-for-TV Movie. However, it proved so popular that a new series was commissioned, and it's been broadcasting more or less continuously to the present day.