The denominations and church bodies I researched were several different English-speaking Lutheran denominations (Lutheran Church - Missouri Synod, Evangelical Lutheran Church in America and the Wisconsin Evangelical Lutheran Synod), the Southern Baptist Convention, the United Church of Christ, the United Methodist Church, the Presbyterian Church in the USA and the Episcopal/Anglican Church. I did try to locate Catholic resources, but the Vatican website was lacking any resources.
The church bodies are listed in order of their domain registrations, with the oldest domains (not the oldest or largest churches) first.
Lutheran Church - Missouri Synod (LCMS)
The Lutheran Church - Missouri Synod was formed in 1847 as the German Evangelical Lutheran Synod of Missouri, Ohio and Other States (they shortened their name on their 100th anniversary) and was the first church organization I researched to register a domain - they registered their domain (lcms.org) in 1992. They currently have 2.5 million baptized members and over 6,150 congregations.
Presbyterian Church in the USA (PCUSA)
The Presbyterian Church in the USA was formed in 1983 as a merger between 2 branches of the church (northern and southern). There currently are 2.3 million members and over 10,000 congregations. They registered their domain (pcusa.org) in 1993.
Evangelical Lutheran Church in America (ELCA)
The Evangelical Lutheran Church in America was formed in 1988 as a merger of 3 other Lutheran church bodies - the American Lutheran Church (ALC), the Lutheran Church in America (LCA) and the Association of Evangelical Lutheran Churches (AELC). Their website states there are 4.8 million members and over 10,500 congregations across the United States and the Caribbean. Their web domain (elca.org) was registered in 1995.
United Methodist Church (UMC)
Organized in 1968, the worldwide membership of the United Methodist Church is 11.2 million members and there are 41,425 congregations. They registered umc.org in 1995
Wisconsin Evangelical Lutheran Synod (WELS)
Founded in 1850, the Wisconsin Evangelical Lutheran Synod comprises 1,829 congregations and almost 500,000 baptized members, worldwide. They registered WELS.net in 1995
Episcopal/Anglican Church (ECUSA)
The Anglican Church traces it heritage back to 1562 and now has 80 million members worldwide in churches that are part of the Anglican Communion. The Episcopal Church in America (ecusa.anglican.org) accounts for over 2 million members and over 7,000 congregations. According to their website, 47% of Episcopal churches maintain websites. They domains anglican.org and episcopalchurch.org were registered in 1995 and 1997, respectively.
Southern Baptist Convention (SBC)
The Southern Baptist Convention formed in 1845 and now consists of over 16 million members and more than 42,000 congregations. They registered sbc.net in 1999
United Church of Christ (UCC)
Organized in 1957, the United Church of Christ has over 1.1 million members and 5,320 congregations. They registered ucc.org in 2005