While other areas of Quito, such as Centro Histórico and the New Town, offer many services -- supermarkets, laundry, internet and banking -- La Mariscal has the most concentrated and easiest to access number of resources.
While these services may cost a bit more, as they are used to gringos and are gringo-cash focused, the convenience is tough to beat. There are laundry services in many hostels (ask when you book a room), though it’s also easy to find them on various Mariscal streets (try on Mariscal Foch, north side of the street just past the Plaza).
Banks like Banco Pichincha, Banco de Guayaquil, and Banco International don’t change currency; however they do have ATMs which accept most major cards (VISA, Plus, Cirrus, Mastercard). Casas de cambio specifically deal with international money, and if you haven’t changed all yours at the airport or gotten enough via cash machine, you can find a couple around the neighborhood (Check out Amazones y Roca).
As far as supermarkets go, La Mariscal has an abundance of small corner store tiendas, the most popular is on the corner of Juan Leon Mera and José Calama. There is even a posh mini-mart called Super Siempre on Foch and 6 de Diciembre. If you’re in need of a fully stocked supermercado, just walk up Lizardo Garcia (east of 6 de Diciembre) to 12 de Octubre and you can’t miss the large red and white sign indicating Supermaxi, Quito’s main grocery chain.
In recent years Internet cafes have opened up across the Mariscal. Today you can find a couple on each block; the Juan Leon Mera/Foch intersection is stacked. Although they all charge under $1 an hour, the quality of the hardware and connection speed vary from place to place. If you find one cafe that doesn't offer satisfactory service, there is probably an alternative right down the street.