From the 4E DMG...
Corridors
1: Straight 4 squares
2: Straight 8 squares
3-5: Ends in door
6: Straight 4 squares, door on right
7: Straight 4 squares, door on left
8: Straight 4 squares, side passage on right
9: Straight 4 squares, side passage on left
10: Three-way intersection ("T")
11: Four-way intersection
12: 90 degree turn left
13: 90 degree turn right
14-15: Ends in chamber (no door)
16: Ends in stairs
17: Straight 4 squares, stairs on right
18: Straight 4 squares, stairs on left
19: Dead end
20: Random encounter
Door Types
1-4: Not stuck
5-8: Wooden
9-11: Barred or locked
12-14: Stone
15-17: Iron
18-20: Wooden portcullis
Beyond a Door
1-2: Corridor left and right
3-8: Corridor straight ahead
9-18: Chamber
19: Stairs
20: False door plus trap
Chamber Size and Shape
1-2: Square 8x8
3-4: Square 10x10
5-6: Rectangle 6x8
7-8: Rectangle 8x10
9-10: Rectangle 10x16
11-12: Octagon 8x8
13-14: Octagon 8x12
15-16: Octagon 12x12
17-18: Irregular, roughly 8x10
19-20: Irregular, roughly 10x16
Chamber Exits
1-5: None
6-11: One
12-15: Two
16-17: Three
18: Four
19: Stairs only
20: One exit plus stairs
For exit other than stairs:
1-10, exit is door
11-20, exit is corridor
Chamber Features:
1-3: Rubble or other difficult terrain
4-5: Crevasse or chasm
6-7: Statue, obelisk, or similar
8-9: Pit
10-13: Pool, fountain, or basin
14-16: Furniture
17-18: Altar, brazier, or arcane symbol on floor
19: Platform or dais
20: Sarcophagus
Stairs
1: Up to dead end
2: Down to dead end
3-9: Down one floor
10-14: Up one floor
15-17: Trapdoor plus ladder up one floor
18-19: Trapdoor plus ladder down one floor
20: Shaft up and down, one floor each way
Just add a Random Encounter Table or a Random Encounter Deck of cards with monsters, traps and hazards. Voila, instant dungeon.