It's not built for high-volume, heavy-duty work, but this HP is an exemplary entry-level sheetfed/flatbed combo scanner for small offices or home offices. It's ideal for digitizing modest stacks ...
While standalone scanners do exist, a lot of the best printers also include the ability to quickly and easily scan documents into your Windows PC or Mac (and sometimes even onto an iPhone or iPad).
Most flatbeds with ADFs will scan legal-size pages via the ADF, but not all do, so be sure to check. You can also find scanners with larger flatbeds, but they will, of course, take up more desk space.