The holders and winners for the past two seasons appeared to be heading for a fourth-round exit at Everton after falling behind to a Louis Saha header.
But Kalou, who had entered the field just five minutes earlier, booked a replay at Stamford Bridge as he finished off a swift counter-attack 15 minutes from time.
It came just moments after Petr Cech had produced a brilliant save to prevent Seamus Coleman doubling the Toffees' lead.
Both sides went close to a late winner as Ramires struck a post for Chelsea and Jermaine Beckford also tested Cech.
The visiting fans had been in good voice, gleefully proclaiming Fernando Torres as "Chelsea's number nine" in reference to the Stamford Bridge club's attempts to sign the Liverpool striker.
But the game lacked urgency at the start with Chelsea's Didier Drogba having the first opportunity for either side when he shot weakly at Tim Howard after five minutes.
Everton's first spell of pressure ended with Leighton Baines delivering a deep cross but Coleman's header back across goal was cleared.
A sliced clearance by Ashley Cole allowed Everton captain Phil Neville to launch a long throw into the box and Diniyar Bilyaletdinov almost profited but the Russian failed to make firm contact.
Chelsea captain John Terry went close to opening the scoring when he headed narrowly over from a Florent Malouda corner after 17 minutes.
Everton came to life midway through the first half as Marouane Fellaini robbed Michael Essien and Saha sent Jack Rodwell through. The youngster got a clear sight of goal but Cech reacted quickly to block his shot.
The hosts had another opportunity moments later as Neville picked out Bilyaletdinov with a cross from the right but his header was comfortably taken by Cech.
Chelsea responded with fit-again Frank Lampard slipping Nicolas Anelka through on goal but the Frenchman was flagged offside.
Anelka timed his run better when Ramires threaded a fine pass through the defence but this time Howard rushed out of goal to block his shot.
Mikel Arteta tried his luck from distance in the closing minute of the half but drilled a low shot wide of Cech's post.
Everton lifted the tempo after the restart and had an early opportunity after Coleman's persistence won possession on the right.
The Irishman fed Rodwell and his cross was almost turned in on the volley by Saha at the back post but Ramires blocked on the line.
Fellaini then brought a fine save from Cech as he met an Arteta cross with a firm header after Cole had needlessly pushed Coleman to concede a free-kick.
Fellaini turned provider to set up Arteta with a shooting chance on the edge of the area with a good nod down but the Spaniard flashed wide.
Everton were rewarded for a good spell of pressure when Saha got behind Terry and rose highest to head home a Baines corner in the 62nd minute.
It maintained Saha's fine personal record against the Londoners, having scored against them three times last season and in the 2009 cup final defeat.
Cech brilliantly prevented Everton from adding a second by saving at point-blank range as Coleman met a teasing Baines cross with a diving header.
The true value of the save became clear moments later as the Londoners broke up the field to equalise.
Kalou, who had seen little of the ball, finished off a swift counter-attack by charging into the area and finding space to hit a low shot into the far corner.
The game then appeared to meander towards a replay but Everton were almost caught out five minutes from as Ramires struck a fierce shot against the post from 25 yards.
Everton substitute Beckford then pounced on a Branislav Ivanovic error to test Cech with a well-struck shot but again the Chelsea keeper saved superbly.