There are not many situations of annexation or unification in recent history, and most of those are accompanied by warfare or guerrilla violence. So this is a relatively unusual situation.
Given the narrow question you asked, the most obvious answer is German unification. The DDR/GDR basically just dissolved and those East German Lander were incorporated into the BRD/FRG under the Basic Law. It didn't take troops invading, so I don't know if you mean it. It was also certainly popular on both sides (at least in the sense of favoring Einheit).
One could also pose the United Arab Republic as an answer, though maybe not one you were looking for. Syria offered to join Egypt in a fit of pro-Nasser pan-Arabism, but also to ward off perceived Communist influence over their government. The hybrid state of two noncontiguous portions only lasted three and a half years until Syria seceded. Nasser talked about pan-Arabism, but didn't make any moves to bully or to woo Syria. The parallel to Crimea is not terribly strong, but it was an annexation (Syria wanted full union, not a federal union) and it did not come from or cause violence.
There are examples of territories annexed to the United States with varying degrees of violence or pacificism, such as the gift of the Northwest Territory by the British (an unexpected windfall), the Louisiana Purchase, Texas Annexation, Oregon Territory from Britain, Mexican Cession & Gadsden Purchase, purchase of Alaska from Imperial Russia, controversial annexation of Hawaii, and so on. I'm not sure the extent to which you'd consider a purchase of territory to fit your criteria, but many of them were done more or less peacefully, except to the extent that the locals suffered violence from the new government. The Mexican Cession came from war, but the Texas Annexation was itself peaceful - excluding the war for independence that Texas had just waged (with a number of border-crossing Americans pushing for the fight).
You could also have examples of territories returned to former sovereigns who lost it from warfare, negotiation, or colonial tribute. This would include the return of Hong Kong from the UK and Macau from Portugal to China, the return of borderland territories from Russia to China, the return of the Sinai to Egypt by Israel, and so forth. I'm not sure if this is what you're looking for, but there are more of these examples. These were all peaceful, but often the previous disposition had been either via warfare or through colonial threats.
There are some other Russian examples mildly parallel to the Crimea. Russia had troops in South Ossetia as observers since 2004 and issued Russian passports to many South Ossetians and Abkhazians during the 2000s. In 2008, this was cited as a basis for Russian troops moving into Abkhazia and strenghtening their position in South Ossetia, though neither has yet been annexed and of course there was a relatively brief period of warfare. The main comparison is that it was a matter of Russia slicing off territory from its neighbors. Russia did something similar with Transnistria, a breakaway portion of Moldova that has Russian troops and a leadership with an essentially Soviet-era outlook. But Russia has not made real moves to annex Transnistria, which is both landlocked and noncontiguous with Russia, so would be an exclave. It is also not ethnically Russian.
You can find other examples where annexation led to non-state violence (terrorism or civil war) like Israel and Jerusalem or Morocco and Western Sahara. One could get into little nitpicking fights about Israel's annexation of the Golan Heights and whether to count its annexation (including Shebaa Farms, claimed by Lebanon) as resulting in violence or as subsumed in the larger regional conflict. None of these annexations have been accepted by an international consensus of states, as with Crimea.