The Hoteliers Association of Siros operates an information booth at the pier in summer; it's open daily from 9am to 10pm. There is a phone, but it is never answered; you'll have to go there. Note that the list of hotels they offer includes only hotels registered with their service. www.syros.com.gr is also useful.
Several banks on the harbor have ATMs. The Ermoupolis hospital (tel. 22810/86-666) is the largest in the Cyclades; it's just outside town to the west near Plateia Iroon. For free luggage storage, ask at Teamwork Holidays (tel. 22810/83-400). The police (tel. 22810/82-610) are on the south side of Miaoulis Square. The port authority (tel. 22810/88-888 or 22810/82-690) is on the long pier at the far end of the harbor, beyond Hotel Hermes. The post office (tel. 22810/82-596) is between Miaoulis Square and the harbor, on Protopapadaki; it's open Monday through Friday from 7:30am to 2pm. The telephone office (OTE) is on the east side of Miaoulis Square (tel. 22810/87-399); it's open Monday through Friday from 7:30am to 3pm. You can get online in the main square, Plateia Miaoulis, at the Internet facility, on the ground floor of the Town Hall, or at Bizanas, a nearby Internet cafe on Plateia Miaoulis; Multirama, at 54 Ethnikis Antistasis, the main drag along the harbor, also has Internet facilities.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.