A decent-sized city, Castro has all the basic services of interest to travelers. Here are a list of the ones you're most likely to need and their locations:
Internet cafés: Sotomayor 204, Latorre 267, as well as many others. Most charge around $.75 per hour.
International call center: 289 Latorre. Rates to the U.S. start from $.20 per minute.
Pharmacies: as is the case throughout Chile, it's hard to walk down the street without spotting at least one. Cruz Verde has two right on the plaza, at San Martín 407, and then on the very next block, at the corner of Blanco and Esmeralda.
Bank/money exchange/travelers checks: BancoEstado (on the plaza, at San Martín 397) will exchange dollars and Euros, though you'll get better rates in Puerto Montt.
Laundry: O'Higgins 438 (note that to enter you'll have to ring the bell).
Post office: next to the Regional Government Building on O'Higgins, across from the plaza.
Hospital de Castro: Freire 852, ph: 632212. They can handle most procedures, though more specialized treatments are more likely to be available in Puerto Montt.
Bikes and kayaks are available for rental from Green Place (Blanco Encalada 31, ph: 85160475).
A helpful tourist information center, smack dab in the plaza, has maps and some small exhibits on local attractions.