Genlias is a project that I'll probably discuss often in this blog. It is the most important online source for Dutch genealogy research. Its goals are amitious:
Dutch archives aim to enter all data from the open civil registers in the Netherlands into Genlias in the near future. These will be supplemented with additional information from older sources and statements of succession.
The Dutch civil register is a register that lists all births, deaths and marriages that took place in The Netherlands since 1811. Most marriages from the period 1811-1922 are now in Genlias (notable exceptions are the marriages from the three largest cities, Amsterdam, Rotterdam and The Hague). If your ancestors came from Holland in the early 20th, or mid- or late-19th century, Genlias should be the first place to go.