Useful site: http://www.eubusiness.com/Euro

The original members of the eurozone were Austria, Belgium, Finland, France, Germany, Ireland, Italy, Luxembourg, the Netherlands, Portugal and Spain. It came into being on 01 Jan 1999, with the fixing of exchange rates between the national currencies of the founder members.

01 Jan 2001: Greece joined, to become the 12th member of the club.

01 Jan 2002: euro notes and coins came into circulation. National currencies were abolished at the end of February.

01 Jan 2007: Slovenia became the 13th member country in a flawless changeover. The former national currency was the tolar (currency code SIT) with an exchange rate of 239.854 tolars to the euro

01 Jan 2008: Malta and Cyprus joined.  EU Finance ministers gave their approval at the meeting held on 10th July  2007.  One euro replaced 0.585274 Cyprus pounds and 0.4293 Maltese lira.

01 Jan 2009: Slovakia has joined, the 16th Eurozone country.  The Slovak koruna is replaced at SKK 30.1260 to 1 EUR

01 Jan 2011: Estonia joined, the 17th country, at a rate of 15.6466 krooni (EEK) = 1 EUR

01 Jan 2014: Latvia joined, the 18th country, at a rate of 0.702804 lats (LVL) = 1 EUR

01 Jan 2015: Lithuania joined, the 19th country, at a rate of 3.4528 litas (LIL) = 1 EUR. This rate had been pegged since 02 Feb 2002. The EU report covering the preparations dated 21/11/2014 is here. 

Latest (Apr 2015): 

Bulgaria, Czech Republic, Hungary and Romania do not currently have a target date for adoption of the euro.

No plans: Denmark, Sweden and the United Kingdom all have an opt-out. The same privilege is not offered to new members of the EU.

Apart from Slovenia which has joined the eurozone, other former Yugoslav republics including Croatia have their own currencies but are de-facto users of the euro for a wide range of transactions.

European micro-states including Monaco, Liechtenstein, Vatican City, San Marino and Andorra are either de-facto users or have adopted the euro as their official currency.

Some former French, Spanish and Portuguese colonies use the Central African Franc, tied on a fixed exchange rate to the euro of 100 CFA Francs = 0.152449

European Commission Economic and Financial Affairs site: http://ec.europa.eu/economy_finance/euro/adoption/who_can_join/index_en.htm