A few years ago I bought some rather unlike-me off-white crepe culottes on sale in Warehouse, wasn't sure if I'd get much use of them but they turned out to be super useful. They were cool in hot weather but covered my legs from blasting office air conditioning when it was hot outside. But they finally got stained last year and I haven't been able to find anything like them.
What really useful or beloved items haven't you been able to replace?