A very good site for beginners and also for advanced researchers is Ahnenforschung.net. The associated forum has a nice and helpful community, where some members have been doing genealogy for up to 50 years. Note that it is a German website and forum, but international guests are very welcome.
For searching in western Poland, the Posen Project is of great importance. There, all 19th century marriages in the former Prussian province of Posen and neighboring areas are indexed and made searchable.
The site Szukajwarchiwach, operated by the Polish government, makes holdings of the national archives accessible. Partly only indexed and for ordering, others already with scans.
With three billion personal records, Familysearch is the largest and also free genealogical database in the world. Run by the Mormons, the church records are the most important component.
With over 150,000 digitized Protestant church records, Archion is the largest provider in Germany. Mostly German, but also entries from the former German provinces can be viewed from home with various paid passes.
Probably the best known worldwide is Ancestry®. As a partially fee-based portal, it offers a wide variety of documents from almost all continents.