Via Elizabeth at Charlottesville Words: The Eggcorn Database.

The word eggcorn was coined collectively by the linguists who write at the excellent group blog Language Log.
Linguists collect usage examples. Unlike language teachers or the often
self-styled grammar experts who complain in the press about the decay
of English, they are not picky: the actual, real-life use is what
counts, and the most interesting bits — those that might reveal
something about how real people apprehend their language — often
stretch the received rules of correctness.

In September 2003, Mark Liberman reported (Egg corns: folk etymology, malapropism, mondegreen, ???)
an incorrect yet particularly suggestive creation: someone had written
“egg corn” instead of “acorn”. It turned out that there was no
established label for this type of non-standard reshaping. Erroneous as
it may be, the substitution involved more than just ignorance: an acorn
is more or less shaped like an egg; and it is a seed, just like grains
of corn. So if you don’t know how acorn is spelled, egg corn actually makes sense.

Other examples of eggcorns are:

"for all intensive purposes"

"just desserts"

"here, here" (instead of "hear, hear")

"coming down the pipe"

The aforementioned Elizabeth gets credit for spotting a new eggcorn in common usage, and it is now included in the dictionary: "half-hazard."

Eggcorns are different from malapropisms, which can also be fun to watch for. A malapropism is a word used in place of the correct word, where the substitution sounds similar to the intended word but means something vastly different, often resulting in quite comical sentences. A famous example is the line uttered by Curly of the Three Stooges: "I resemble that remark!"

(The link takes you to WikiPedia, where there are many more examples of malapropisms and eggcorns, including a malapropism that made me laugh out loud: "New Scientist also reported the first-ever malapropism for
"malapropism", when, having become aware of his error, the office
worker apologized, saying he had committed a "Miss Marple-ism."  No doubt he was thinking of Mrs. Malaprop, the character in Richard Brinsley Sheridan’s play, The Rivals, whose comical linguistic errors gave rise to the term.)

So a malapropism is a wrong word used in place of a word that sounds similar, but not identical, and has a totally different meaning. An eggcorn is a substitution that sounds the same or almost the same as a word or phrase—so similar, and making just enough sense, that it often passes into common usage: "blatantly obvious" instead of "patently obvious."

Don’t you just love the English language?