The American Heritage Dictionary, The AP Stylebook, The Chicago Manual of Style, and The New York Times Manual of Style and Usage suggest using a colon to set off lengthy quotations.
The Chicago Manual of Style and The New York Times Manual of Style and Usage suggest using a colon before a quotation that uses no quotation marks.
The AP Stylebook and the Merriam-Webster Guide to Punctuation suggest using a colon to set off a long indented quote.
The AP Stylebook and the Merriam-Webster Guide to Punctuation also suggest using a colon to set off a regular quote.
The MLA Style Manual suggests using a colon to set off a quote of independent structure.