(I realise scene-related info is ever-changing. This is a snapshot made on 2016-03-07. Furthermore, I may be slightly C64-biased, since that is what I used back in the day. :-)
@party (Somerville, Massachusetts) - seems to be mostly modern, but also at least Apple 2
1989 - Over de muur! (Utrecht, the Netherlands) - Amiga, C64
Alternative Party (note "2014" in URL) (Wikipedia) (Finland) - Less common computers, arts, music, graphics. Still ongoing?
Assembly (Wikipedia) (Finland) - Winter: gaming; summer: demos. "Oldskool" compo for older machines (which?)
BCC (site in German) (Berlin, Germany) - C64, other 8-bit welcome
Birdie (site in Swedish) (Uppsala, Sweden) - seems to be mostly/only for modern machines
Chaos Constructions (Saint Petersburg, Russia) - PC, Amiga, ZX Spectrum
Datastorm (Gothenburg, Sweden) - copyparty, with compos for PC, Amiga, C64
Decrunch (Wroclaw, Poland) - Amiga, Commodore, Atari, ZX Spectrum, others
Demosplash (Pittsburgh, Pennsylvania) - modern, as well as Amiga, Amstrad, Apple, Atari, Commodore, Sinclair, Tangerine, TI, Vectrex
DiHalt (lite) (Wikipedia, Russian) (Nizhny Novgorod, Russia) - ZX Spectrum. PC, Amiga, C64
DreamHack (Wikipedia); location varies (2016: Austin, Texas) - largest digital festival, gaming, digital art, concerts, expo (demos only seem to be a very small part)
Evoke (Cologne, Germany) - PC, and optionally alternative/oldskool platforms
Function (Budapest, Hungary) - PC, Amiga, C64, other?
Forever (site 1, site 2) (Trencin, Slovakia) - Atari, C64, ZX Spectrum, Amstrad, others
The Gathering (site in Norwegian; demo-stuff in English is here, or Wikipedia) (Norway) - may be more game-oriented, but demo-compo(s) for PC, Amiga, Atari, C64, Nintendo
Instanssi (Jyvaskyla, Finland) - beginner-friendly, perhaps mostly/only for new machines
Kindergarden (Wikipedia) (Norway) - Pure demo-scene, PC, Oric, C64, various. (Not held in 2015; cannot find 2016 invite/info.)
LayerOne demoparty within security conference (Los Angeles, California) - PC, others (bring hardware or emulated, e.g. C64, Amiga, ZX Spectrum)
Multimatrograf (Vologda, Russia) - ZX Spectrum, C64, Atari, Amstrad, MSX, BK, PC/XT
Nordlicht (Bremen, Germany) - PC, Amiga, C64, Atari, others
Nullarbor (site down ATM? 2016-03-07; see Wikipedia) (Perth, Australia) - game-development and demo-scene
Outline (Willemsoord, the Netherlands) - PC, Atari, Amiga, C64, perhaps others
QBparty (Sulysap, Hungary) - C64, Amiga, others (bring hardware)
Reset (Coutances, France) - Amstrad only
Revision (Wikipedia) (Saarbruecken, Germany) - PC, Amiga, oldskool (C64, VIC20, Atari XL/ST, (S)NES, ZX Spectrum, ...)
Rewired (Wikipedia) ((Maaseik?,) Belgium) - PC, C64, ..?
Silly Venture (Gdansk, Poland (always?)) - Atari
Simulaatio (Joroinen, Finland) - PC, Amiga, C64, others?
Solskogenhttp://www.solskogen.no/ (outside of Oslo, Norway) - PC, C64, Amiga, Atari, wild
Sundown (Budleigh Salterton, England) - PC and oldskool (which?)
Synchrony (Manhattan, New York) - PC, Atari VCS, VIC20, C64, Amiga, (S)NES, ZX Spectrum, ...
Syntax Party (Melbourne, Australia) - PC, C64, VIC20, OCS, ECS, ST, AGA, Spectravideo, others
Tokyo Demo Fest (Tokyo, Japan) - seems to be mostly/only PC
The Ultimate Meeting (Wikipedia (German)) (Griesheim, Germany) - canceled in 2014 - held in 2015? which platform(s)?
Vintage Computer Festival Europe (German site or English one, which seems a bit outdated) (Muenchen, Germany) - not a demo-party but more of a general gathering, it seems
Very Important Party (note "2014" in URL; also see Wikipedia) (Saint-Priest, France) - PC, oldskool (which?)
Wap-Niak (site in Polish) (Warsaw, Poland) - Atari, ZX Spectrum, others?
X (Wikipedia) (Someren, the Netherlands) - C64