Which official LEGO sets are micro-scale versions of existing sets?
(M) = Microscale
Harry Potter

4841 Hogwarts Express,
(M) 40028 Mini Hogwarts Express
4755 Knight Bus,
(M) 4695 Mini Harry Potter Knight Bus

Modular

10182 Cafe Corner,
10190 Market Street,
10185 Green Grocer,
10197 Fire Brigade,
10211 Grand Emporium,
(M) 10230 Mini Modulars

Pirates of the Caribbean

4184 The Black Pearl,
(M) 30130 Mini Black Pearl

Star Wars

Various TIE Fighter, (M) 3219 Mini TIE Fighter
Various Millennium Falcon (M) 4488 Millennium Falcon and (M) 75030 Millennium Falcon
Various AT-AT Walker, (M) 4489 AT-AT
7675 AT-TE Walker,
(M) 20009 AT-TE Walker
8098 Clone Turbo Tank,
(M) 20006 Clone Turbo Tank
7163 Republic Gunship,
(M) 20010 Republic Gunship
7674 V-19 Torrent,
(M) 8031 V-19 Torrent
Lots of Mini's in the Advent Calendars, 7958 2011, 9509 2012
10019 Rebel Blockade Runner,
(M) 10030-1 Imperial Star Destroyer contains micro version

Technic

8053 Mobile Crane,
(M) 8067 Mini Mobile Crane
8275 Motorized Bulldozer,
(M) 8259 Mini Bulldozer
8295 Telescopic Handler,
(M) 8045 Mini Telehandler
8069 Backhoe Loader, (M) 42004 Mini Backhoe Loader

Creator

31003 Red Rotors, (M) 30184 Little Helicopter
31004 Fierce Flyer, (M) 30185 Little Eagle
31020 Twinblade Adventures, (M) 30189 Transport Plane
6743 Street Speeder, (M) 6910 Mini Sports Car

Misc.

10226 Sopwith Camel, (M) 40049 Mini Sopwith Camel
10220 Volkswagen T1 Camper Van, (M) 40079 Mini VW T1 Camper Van

Possible

10194 Emerald Night, (M) 31015 Emerald Express
8110 Mercedes Benz Unimog U 400, (M) 9390 Mini Tow Truck