An eggcorn is a new label for a spontaneously reshaped known expression.  


It's when a common expression like "free rein" is semantically re-analyzed and then re-written to reflect the new analysis, like "free reign".  Eggcorns aren't just misspellings.  Nor are they examples of folk etymology (e.g. "Jerusalem artichoke" for "girasole artichoke"), malapropism (e.g. "fortuitous" for "fortunate") or mondegreens (e.g. "Excuse me while I kiss this guy"). 

This website has a database of eggcorns with sample citations and a brief description.

Some of my favorites are: to besiege someone rather than beseeching them, to get balked down rather than getting bogged down, whorefrost for hoarfrost, rebel rousing instead of rabble rousing, and the self-congratulatory whoa is me for woe is me.  (Some of these may be simple misspellings.)