Both the Smithsonian website and the website for VolcanoDiscovery post details on recent volcanic eruptions worldwide, while the U.S. Geologic Survey's Volcano Hazards website details eruptions within the United States. There are also several volcano observatories that post data online, but only for a limited number of locations.
The Smithsonian's Global Volcanism Program produces a webpage in conjunction with the USGS that details volcanic activity worldwide, week by week. The webpage on the Smithsonian website lists both new eruptions and ongoing lava flows.
VolcanoDiscovery provides a map and list of currently active volcanoes. Locations are color-coded to distinguish between major eruptions and less dramatic events, such as volcanic unrest.
The USGS maintains a website for the Volcanic Hazards Program. The site lists many resources on volcanism, including news about recent eruptions and links to volcano observatories. One such observatory, the AVO of Alaska, also maintains a website of its own, with information on recent eruptions in Alaska and surrounding areas.