Currently erupting:

Ambrym (Vanuatu): active lava lakes in several craters (updated 1 Sep 2017)
Aoba (Vanuatu): ash venting, phreatomagmatic explosions (updated 28 Nov 2017)
Bagana (Bougainville Island, Papua New Guinea): intermittent mild ash emissions (updated 10 May 2017)
Bezymianny (Central Kamchatka Depression): new activity since around 12 Dec (updated 20 Dec 2017)
Dukono (Halmahera): mild strombolian activity, continuous intense ash emissions (updated 28 Nov 2017)
Erebus (Antarctica): active lava lake in summit crater (updated 8 Dec 2014)
Erta Ale (Danakil depression, Ethiopia): lava lake in summit south crater rises to 35 m below rim and very active (updated 6 Dec 2017)
Fuego (Guatemala): lava flows on S and W flanks, lava fountaining from summit vent (updated 9 Feb 2018)
Heard (Australia, Southern Indian Ocean): lava effusion at summit vent (updated 22 Jun 2017)
Ibu (Halmahera, Indonesia): stromolian and phreatomagmatic explosions (updated 29 Oct 2015)
Kadovar (Northeast of New Guinea): continuous explosions, lava flow emission from fissure vent (updated 19 Jan 2018)
Karymsky (Kamchatka): weak activity (rare explosions?) (updated 23 Jul 2017)
Kilauea (Hawai'i): new surface lava flows from breakouts N and NE of Puu Oo since 24 May 2016 (updated 29 Sep 2016)
Kirishima (Kyushu): increased degassing at Iwo crater, seismic unrest (updated 5 Apr 2018)
Klyuchevskoy (Kamchatka): mild activity at summit crater, occasional explosions (updated 23 Jun 2017)
Masaya (Nicaragua): active lava lake ini summit crater (updated 12 Jul 2016)
Nyiragongo (DRCongo): new vent on crater floor (updated 9 Sep 2016)
Ol Doinyo Lengai (Tanzania): effusion of natrocarbonatite lava inside the pit crater in the northern main crater (updated 7 Dec 2017)
Pacaya (Guatemala): strombolian activity in summit crater, lava flow effusion (updated 2 Mar 2017)
Piton de la Fournaise (La Réunion): eruption from south flank of Dolomieu crater since 14 July 2017 (updated 3 Apr 2018)
Popocatépetl (Central Mexico): degassing, sporadic explosions, slowly growing lava dome (updated 1 Dec 2017)
Reventador (Ecuador): lava flow on SW flank (updated 1 Dec 2017)
Sabancaya (Peru): frequent moderate explosions, intense ash emissions (updated 1 Dec 2017)
Sakurajima (Kyushu, Japan): strombolian activity at Showa crater (updated 5 Apr 2018)
Sangay (Ecuador): mild strombolian activity in summit crater (updated 7 Aug 2017)
Santiaguito (Guatemala): intermittent strong explosions from growing Caliente dome (updated 9 May 2017)
Semeru (East Java, Indonesia): growing lava dome, mild strombolian activity in summit crater (updated 10 Oct 2016)
Shiveluch (Kamchatka): growing lava dome (updated 5 Dec 2017)
Sinabung (Sumatra, Indonesia): continuing pyroclastic flows (updated 23 Feb 2018)
Stromboli (Eolian Islands, Italy): intense strombolian explosions, occasional lava overflows from vents (updated 10 Mar 2018)
Villarrica (Central Chile): small active lava lake in summit crater (updated 23 Aug 2016)
Yasur (Tanna Island, Vanuatu): ash emissions, weak strombolian explosions (updated 16 Dec 2017)

Eruption warning / minor activity:

Agung (Bali): new lava dome growing in summit crater (updated 13 Feb 2018)
Aso (central Kyushu, Japan): degassing, sporadic steam / ash explosions (updated 8 Oct 2016)
Awu (North Sulawesi & Sangihe Islands): seismic unrest, eruption warning (updated 13 May 2016)
Bogoslof (United States, Aleutian Islands): quiet since 13 March 2017 (updated 8 Aug 2017)
Bristol Island (United Kingdom, South Sandwich Is): eruption seems to have ended (updated 14 Sep 2016)
Bromo (East Java, Indonesia): intense degassing, intermittent ash emissions, crater glow (updated 10 Oct 2016)
Bulusan (Luzon Island, Philippines): intermittent ash emissions, intense degassing (updated 29 Dec 2016)
Canlaon (Central Philippines): degassing, sporadic ash emissions (updated 6 May 2017)
Cerro Negro (Nicaragua): increased earthquake activity (updated 16 Feb 2016)
Chaitén (Southern Chile and Argentina): seismic unrest (updated 22 May 2015)
Chikurachki (Paramushir Island): steaming, degassing (updated 23 Aug 2016)
Chirinkotan (Northern Kuriles, Russia): possible large explosion on 28 Nov 2016 (updated 2 Mar 2017)
Chirpoi (Kurile Islands, Russia): degassing, steaming (updated 29 Jul 2016)
Cleveland (Aleutian Islands, Alaska): new lava dome growing in summit crater (updated 4 Apr 2018)
Copahue (Chile/Argentina): mild ash and steam emissions (updated 28 Nov 2017)
Cotopaxi (Ecuador): steaming (updated 25 Aug 2016)
Dieng (Central Java): alert level back to normal since 9 Aug 2014 (updated 3 Jul 2017)
Ebeko (Paramushir Island): sporadic explosions (updated 28 Nov 2017)
Etna (Sicily, Italy): mini-eruptions at SE crater at regular intervals (updated 19 Apr 2014)
Karangetang (Siau Island, Sangihe Islands, Indonesia): rockfalls, glowing lava dome (updated 13 Jan 2016)
Katmai (Alaska Peninsula, USA)
Kerinci (Sumatra): intermittent ash explosions (updated 5 Jun 2016)
Krakatau (Sunda Strait, Indonesia): unrest, eruption pause or eruption ended (updated 22 Feb 2017)
Kusatsu-Shirane (Honshu): seismic unrest (updated 24 Jan 2018)
Langila (New Britain, Papua New Guinea): likely lava flows or lava dome extrusion, strombolian activity (updated 23 Jun 2017)
Lascar (Northern Chile): sudden explosion on 30 Oct 2015 (updated 31 Oct 2015)
Loihi (United States, Hawaiian Islands): earthquake swarm (updated 26 Jun 2017)
Lokon-Empung (North Sulawesi, Indonesia): seismic unrest, eruption warning (updated 9 Mar 2016)
Lopevi (Vanuatu): probably strombolian activity in summit crater (updated 13 Jan 2017)
Manam (Papua New Guinea): mild explosive activity at summit crater (updated 8 May 2017)
Marapi (Western Sumatra, Indonesia): steaming, sporadic small explosions (updated 5 Jun 2017)
Mayon (Luzon Island, Philippines): effusion of viscous lava in summit crater, lava flow reaching foot of volcano (updated 26 Feb 2018)
Merapi (Central Java, Indonesia): sporadic phreatic explosions (updated 21 Aug 2015)
Michael (United Kingdom, South Sandwich Is): heat signal from summit crater, unknown eruptive activity (updated 28 Nov 2016)
Momotombo (Nicaragua): intermittent vulcanian explosions (updated 29 Mar 2016)
Nevado Chachani (Peru)
Nevado del Ruiz (Colombia): intermittent mild to moderate ash emissions (updated 18 Oct 2016)
Nevados de Chillán (Central Chile): sporadic mild to moderate ash emissions (updated 2 Apr 2017)
Niigata-Yake-yama (Honshu): sporadic explosions, seismic unrest (updated 8 May 2016)
Nyamuragira (DRCongo): active lava lake in summit caldera (updated 2 Feb 2017)
Poas (Costa Rica): intermittent low-energy ash emissions, intense degassing (updated 14 Jul 2017)
Rincón de la Vieja (Costa Rica): sproadic hydrothermal explosions (updated 23 Jun 2017)
San Cristobal (Nicaragua): intermittent ash explosions (updated 24 Apr 2016)
San Miguel (El Salvador): seismic unrest, inflation, strong degassing (updated 9 May 2017)
Sangeang Api (Indonesia): strombolian activity at summit vent, lava flow effusion (updated 9 Feb 2018)
Soufriere Hills (Montserrat, West Indies (UK)): seismic unrest, occasional light ash venting (updated 21 Mar 2013)
Suwanose-jima (Ryukyu Islands): intense strombolian activity from Otake crater (updated 10 May 2017)
Telica (Nicaragua): lava glow from crater, degassing, occasional light ash emissions (updated 9 May 2016)
Tinakula (Santa Cruz Islands, Solomon Islands): increased activity at the volcano (updated 8 Feb 2013)
Tungurahua (Ecuador): inflation, eruption warning (updated 5 Jun 2016)
Turrialba (Costa Rica): sporadic mild ash emissions (updated 23 Jun 2017)
Ubinas (Peru): intermittent ash explosions (updated 4 Oct 2016)
Ulawun (New Britain, Papua New Guinea): degassing, ash venting (updated 5 Aug 2013)
Zavodovski (South Sandwich Islands (UK)): strong steaming, degassing (updated 18 Jun 2016)