Consonant harmony is a type of "long-distance" phonological assimilation akin to the similar assimilatory process involving vowels, i.e. vowel harmony.
A good discussion of consonant harmony typology is found in Rose and Walker's 2004 paper in the journal Language, "A Typology of Consonant Agreement as Correspondence."
One of the more common harmony processes is coronal harmony, which affects coronal fricatives, such as s and sh. Then, all coronal fricatives belong to the +anterior class (s-like sounds) or the -anterior class (sh-like sounds). Such patterns are found in the Dene (Athabaskan) languages such as Navajo (Young and Morgan 1987, McDonough 2003), Tahltan (Shaw 1991), Western Apache, and in Chumash on the California coast (Applegate 1972, Campbell 1997). In Tahltan, Shaw showed that coronal harmony affects three coronal fricatives, s, sh and the interdental th. The following examples are given by de Reuse: in Western Apache, the verbal prefix si- is an alveolar fricative, as in the following forms:
However, when the prefix si- occurs before a verb stem that contains a post-alveolar affricate, the si- surfaces as the post-alveolar shi-:
Thus, all sibilant obstruents (fricatives and affricates) in these languages are divided into two groups, +anterior (s, ts, dz) and -anterior (sh, ch, j). In Navajo, as in most languages with consonant harmony, there is a constraint on the shape of roots (a well-formedness constraint) that is identical to the harmony process. All roots with sibiliant affricates or fricatives have the same value for anteriority. Shaw (1991) provides a phonological analysis of this process, using data from research on Tahltan.
There are two interesting aspects of the process in Navajo. Firstly, morphemes that participate are domain-specific, only the last two domains are affected (conjunct + stem). Verbal morphemes from the outer or 'disjunct' domain are not affected by the process: the process is morphologically conditioned. Secondly, the lateral affricate and fricative (dl, tł and ł) appear with both values. Young and Morgan (1987) offer an extensive sets of examples of this type of morpheme alternation in Navajo.
A different example of coronal harmony occurs in Sanskrit, [n] being retroflexed to [ɳ] if certain consonants precede it in the same word, even at a distance.
Various Austronesian languages have consonant harmony among the liquid consonants, with [r] assimilating at a distance to [l] or vice versa.
Guaraní shows nasal harmony, and certain affixes have alternative forms according to whether the root includes a nasal (vowel or consonant) or not. For instance, the reflexive prefix is realized as oral je- before an oral stem like juka "kill" but as nasal ñe- before a nasal stem like nupã "hit." The ã makes the stem nasal.
Some Finnish-speakers find it hard to pronounce both 'b' and 'p' in loanwords ( pubi, pub) and so they voice (bubi) or devoice (pupi) the entire word. It should however be noted that the distinction between the consonants is not native to Finnish. Native Finnish words do not use the letter 'b'.