An introduction, often on stage, indicating that the person announced last is no less important than those introduced earlier.

We know the phrase best from its use in the theatre. In Variety theatre in particular it was a commonplace part of introductions and that usage was presumably encouraged by the fact that the star turn invariably came on last. The origin of last but not least is uncertain - the first reference to it that I can find in print is from The Edinburgh Advertiser, May 1824:

"The Government had afforded relief to the humbler classes, by taking off the greater part of the salt tax, half of the leather tax and last, but not least, the lottery."

The idea, if not the actual phrase, may be taken from the Bible. In Matthew 19:30 (King James Version), we find:

"But many that are first shall be last; and the last shall be first."

Shakespeare also used a version of the phrase in King Lear:

KING LEAR:

To thee and thine hereditary ever
Remain this ample third of our fair kingdom;
No less in space, validity, and pleasure,
Than that conferr'd on Goneril. Now, our joy,
Although the last, not least; to whose young love
The vines of France and milk of Burgundy
Strive to be interess'd; what can you say to draw
A third more opulent than your sisters? Speak.