I would concur with the above post, the most recent addition I can source is Montenegro (admitted on 28 June 2006).
There is some speculation that Kosovo, an autonomous province of Serbia which has declared independence, is not expected to be admitted to the UN in the near future due to Russia's refusal to recognize Kosovo, as admission to the UN requires approval from the Security Council, and Russia, being one of the five permanent members of the Security Council, has veto power.