I would like to recommend this cardigan. It costs £25, comes in grey, black, brown, purple or blue and is machine washable (well it says dry clean only on the label but I've washed mine loads of times). It's a really fine knit and a great length. I got the black last year and have just bought the brown one. It's good over jeans, dresses and smart clothes. Really, really useful.
What have you got that's your wardrobe staple?