Gareth Barry's late penalty earned Villa a draw in an incredible game.
Barry celebrates Villa's equaliser as Cech gazes on
Shaun Maloney volleyed Villa ahead and Petr Cech's gaffe gifted him a second but Zat Knight saw red and Andriy Shevchenko netted from the spot.
Shevchenko spanked Chelsea level and Alex crashed in a third but Martin Laursen equalised before Ricardo Carvalho saw red for a nasty lunge.
Michael Ballack's free-kick seemed to have won it but Barry made it 4-4 when Ashley Cole was sent off for handball.
At 2-0 down with seconds to go before the interval, an out-of-sorts Chelsea would probably have been thrilled with a point and the chance to extend their 71-game home unbeaten league run.
But having played the entire second half against ten men and twice led in the final 15 minutes, boss Avram Grant will be furious with his side's failure to protect their lead.
The tone was set from the outset as Chelsea made a sloppy start and early mistakes by full-backs Ashley Cole and Paulo Ferreira saw Ashley Young and Gabriel Agbonlahor drag efforts narrowly wide.
Villa's wing duo were the chief tormentors of Chelsea in their victory over Jose Mourinho's Blues at Villa Park earlier in the season.
And it was no surprise when Agbonlahor worked his magic to tee up the away side's thoroughly deserved opener.
Agbonlahor robbed Salomon Kalou and worked some space on the right before finding the imposing figure of John Carew and his clever header back across goal was volleyed in by Maloney.
Chelsea were struggling to deal with the pace and direct running of Villa's wingers and Agbonlahor had two further chances.
He blazed the first over from a tight angle and saw a deflected shot well kept out by Cech.
Chelsea's cause was hardly helped by a thigh injury to the inspirational Frank Lampard, although it did allow Ballack to come on for his first taste of Premier League action since April.
The Blues's only noteworthy chance of the opening half an hour came from the boot of Joe Cole but his weak effort was easily saved by Scott Carson.
But just as the Londoners began to look like getting back into the match, Maloney doubled Villa's lead courtesy of a terrible blunder by Cech.
The former Celtic striker took aim from 20 yards and Cech went down on one knee to gather but got his angles all wrong and the ball squirmed up off his arm and into the corner of the net.
At 2-0 and with Chelsea all at sea and Villa in control and full of confidence, the game looked as good as over.
Shevchenko celebrates his equaliser
But the whole complexion changed deep into stoppage time.
Michael Essien scuffed a shot from 25 yards and the ball fortuitously fell to Ballack who got in front of Knight and was bundled over before he could reach the ball.
Referee Phil Dowd sent off Knight and Shevchenko coolly sent Carson the wrong way from the penalty spot.
Shevchenko then had a glorious chance to level within two minutes of the restart but he shot tamely at Carson from the edge of the box.
However he did not have to wait long.
Kalou wriggled free on the edge of the box and laid the ball across to Shevchenko who took one touch and spanked home a vicious shot that deceived Carson in the air.
Chelsea were rampant and the pressure was relentless.
Kalou blazed over, Ballack twice went close from 20 yards but it was Alex who put Chelsea ahead.
The Brazilian centre-half Alex brought the ball out of defence, exchanged passes with Shevchenko and showed neat footwork before fizzing a low shot into the bottom corner.
Villa though were not beaten and Laursen soon restored parity when he guided a volleyed effort home from Young's beautifully curled free-kick.
Carvalho then saw red for a terrible two-footed lunge on Agbonlahor to level up the number of players.
Both teams were still going for the victory and it was the hosts who seemed to have got it when Ballack stroked home a wonderful 87th-minute free-kick.
Amazingly there was still time for Villa to level.
Villa kept the ball alive in the box and Agbonlahor's header was cleared off the line by Ashley Cole.
Cole claimed it was headed away but Villa screamed penalty for handball and referee Dowd agreed.
And Barry showed great composure to earn Villa a deserved point from the spot and put a huge dent in Chelsea's title aspirations.
Chelsea: Cech, Ferreira, Alex, Carvalho, Ashley Cole, Kalou, Essien, Lampard (Ballack 26), Joe Cole, Pizarro (Obi 85), Shevchenko (Wright-Phillips 82).
Subs Not Used: Hilario, Ben-Haim.
Sent Off: Carvalho (80), Ashley Cole (90).
Booked: Essien, Alex, Ballack.
Goals: Shevchenko 45 pen, 50, Alex 66, Ballack 88.
Aston Villa: Carson, Mellberg, Laursen, Knight, Bouma (Harewood 90), Maloney (Davies 46), Reo-Coker, Barry, Young, Carew (Moore 78), Agbonlahor.
Subs Not Used: Taylor, Gardner.
Sent Off: Knight (45).
Booked: Reo-Coker, Harewood.
Goals: Maloney 14, 44, Laursen 72, Barry 90 pen.
Ref: Phil Dowd (Staffordshire).
BBC Sport Player Rater man of the match: Aston Villa's Shaun Maloney 8.30 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.