Examples from relatively familiar languages are the ɓ ɠ of Zulu and the ɓ ᶑ ʄ ɠ of Sindhi (the second of these is retroflex). According to Ladefoged and Maddieson (The Sounds of the World’s Languages, Blackwell 1996), the Igbo labialvelar spelt gb, often considered a voiced pulmonic plosive with double articulation, i.e. simultaneous b and g, is more accurately described as a voiceless bilabial implosive, ɓ̥.
In the WALS data, implosives are found in 75 of the 567 languages studied (= 13%), most of them in Africa or southeast Asia. (Ladefoged and Maddieson, however, say that implosives are found in “about 10% of the world’s languages”.) By far the commonest implosive is the voiced bilabial ɓ.
The OED reveals that as a phonetic term ‘implosive’ goes back at least as far as Sweet, who wrote in 1890 that
Some sounds are produced without either out- or in-breathing, but solely with the air in the throat or mouth. The ‘implosives’ are formed in the former, the suction-stops or ‘clicks’ in the latter way.
There is one other usage you may occasionally come across in older works, in which an ‘implosive’ stop is an ordinary pulmonic stop with no audible release stage, or in which the release is not taken into account, as against an ‘explosive’ one which has no audible approach stage, or in which the approach stage is not taken into account. The OED has a quotation from Bazell in 1953:
If all initial occlusives are explosive and all final occlusives are implosive, it is obvious that two distinct conventions (explosiveness of initials and implosiveness of finals) need not be postulated.We can regard this meaning as obsolete.
We pair implosives with ‘ejectives’, sounds made with a glottalic egressive airstream mechanism. They have an airstream initiated by the raising of the closed glottis, which compresses the air in the supraglottal cavity. The OED’s first citation for this term is Daniel Jones in 1932.
In IPA ejectives are written with an apostrophe diacritic, thus pʼ tʼ kʼ etc. They are “not at all unusual sounds, occurring in about 18 percent of the languages of the world” (L & M). Although they are not contrastive in English or any other European language, many speakers in Britain use ejectives as optional phrase-final variants of p t k.
I made a Google ngram for these two terms. Rather mysteriously, it shows