Nordic countries

Norway

Norway disestablished the state church, the Evangelical Lutheran Church of Norway, in 2012 but this is largely symbolic since its financial privileges are retained.

The option of supporting religious bodies through membership dues was rejected and state funding is to continue. Norway funds the Lutheran Church — and the others — through general tax revenues. It offers the same amount of money per member to each religious denomination. However, as citizens are considered to be members of Lutheran church, unless they explicitly state otherwise, it profits from being the default option. And although the King is no longer formally the head of the Lutheran Church with the right to nominate its bishops, the ruler is still obliged to be a Lutheran. Furthermore, the state keeps on paying the salaries of the Lutheran clergy, their pensions and the upkeep of their churches.
 

Denmark

Danes who are baptised in the Church of Denmark, the national Lutheran church, must pay the tax as long as they are members. [...] If a currently registered church member wants to opt out of membership, they must write to their local church office and request a resignation of membership form in addition to providing their birth certificate. Once a person opts out of the Church of Denmark, they can no longer have a minister of that church preside over religious events like a wedding or funeral. (Copenhagen Post)

The Danish website Ingen kirkeskat ("No church tax") has a form where you can calculate how much you'd save by leaving.
 

Finland

The country has two state churches, Lutheran and Orthodox. The larger Lutheran state church has about 80% of the population, the smaller Orthodox one, about 1%. Those wishing to avoid being taxed by these can visit the website Eroakirkosta which roughly translates as "Leave-the-church". There they can download a church-leaving form to send to the government, or even do it by email right on this site. It does not cover non-state churches like the Catholic Church.

A recent spike in Finns leaving the Lutheran Church appears to have had less to do with church tax than with a reaction to the social doctrines of the church. On 12 October 2010 the Finnish Broadcasting Corporation featured a TV panel discussion on gay rights featuring both the Bishop of Tampere and a Christian Democrat MP who supports church opposition to same-sex marriages and adoption rights for registered same-sex couples. That day a record number of Finns voted with their feet. 2,633 left the Lutheran Church which was about 1,500 more than the previous daily high.

Sweden

Until 2000 there was a mandatory church tax collected for the Lutheran state church. After the church was disestablished on New Year's Day, the government has agreed to continue collecting church tax, but has made it optional. Now there is even a website with an email form for leaving the church.

Iceland

Individuals are free to direct their church tax payments to any of the religious groups officially registered and recognized by the State. For individuals who are not registered as belonging to a religious organization, or who belong to one that is not registered officially and recognized by the State, the tax payment goes to the University of Iceland, a secular institution. (Iceland: International Religious Freedom Report 2004)

There is also an Icelandic church-leaving website.