Sure. The United Arab Republic springs to mine, although it didn't exactly last very long. Also the unification of North and South Yemen. The United Arab Emirates was a voluntary confederation of most of the former Trucial States. Those are the big ones I can think of in my particular section of the world, and I'm sure there are others. Tanzania forming through the unification of Tanganyika and Zanzibar, for instance. Unfortunately I think this question could fall afoul of "trivia seeking" on that basis.

When India was granted independence in 1947, there were a number of princely states (Bhopal, Hyderabad, Travancore, for e.g.) within the former British India that were independent from the Union of India and from the newly formed Pakistan. Over the next few years, they were one by one assimilated either into India or into Pakistan. Some went willingly, others had to be persuaded/threatened. The most infamous example of this is Kashmir which is still a cause for enmity between India and Pakistan 60+ years later.

Source: India After Gandhi by Ramachandra Guha