Some links to flight planning sites or products that I’ve found useful.
Autorouter – The ultimate flight planning tool for IFR in Europe. Provides automatic route finding to handle the byzantine routing rules found in most of Europe. It will generate a complete briefing pack with ATS and operational flight plan, MET and AIS information, approach plates, fuel plan, performance and W&B calculations. As a bonus you get access to a chatbot for handling flight plans and slots, getting up-to-date MET and NOTAM information… Access is free for pilots, but you need to register.
European AIS Database (EAD) – The official European repository for AIS data. Here you can find the AIP of every European country (and some more). You can also get tailored NOTAM briefings. Access is free for pilots, but you need to register.
NorthAviMet – An official aviation meteorology web site run jointly by the MET offices of the eight Nordic and Baltic countries. Access is free for pilots, but you need to register.
SkyDemon – An excellent flight planning and navigation software, primarily intended for VFR but also quite useful for IFR. Among other things it provides access to AIP plates and (for an additional subscription fee) several national VFR airport manuals.
Windy – An amazing weather forcast visualisation site, including data important for aviation such as cloud base, temperature and wind aloft etc. You can choose between several base forecasts such as the US GFS or European ECMWF and ICON.