The current editions of Merriam-Webster and the Oxford English Dictionary are available online, and undergo continuous revision. But they have professional editorial teams keeping them up to date, and so a subscription is required. Their 20th-century printed editions, that is, the Merriam-Webster 2nd edition (1934), and the OED 1st edition (1928, supplement 1933), are now out of copyright. You will find links to scanned copies at the Internet Archive Digital Library in the menu. Google Sites will warn you that you are being directed off-site. The site archive.org belongs to a well-respected non-profit and the warning is a formality.
Despite the date, the first edition of the OED is in reality very much a 19th-century dictionary. Work began on it in 1857 and the first unbound instalments begin to appear in 1884.