Millimeter - Complete 12 normal levels
Just play the game

Centimeter - Complete 24 normal levels
Just play the game

Decimeter - Complete 36 normal levels
Just play the game

Meter - Complete 45 normal levels
Just play the game

Decameter - Complete all bonus levels
Just play the bonus levels

Kilometer - Complete all normal levels
Finish all 48 normal levels.
Spoiler:
You unlock the last three by getting all prisms in previous levels

A-list - Get an A rank on all normal levels
Self-explanatory

Stardom - Get an S+ rank on all normal levels
Self-explanatory

Rainbow - Collect all prisms in the normal levels
Self-explanatory

Double Rainbow - Collect all prisms in the game
Self-explanatory

Colorblind - Finish a level without collecting any prisms
Spoiler:
Can be done quite easily on level 10 - "metro". Just avoid any cubes and head straight for the exit (Thx to mcwizardry)

Microsecond - Get 10 seconds of EDGE time as a mini cube
Spoiler:
Best done on level 24 - "higher"

Second - Get 10 seconds of EDGE time
Can be completed on any stage

Minute - Get 30 seconds of Edge time
Can be completed on any stage

Hour - Get 60 seconds of EDGE time
Can be completed on any stage

Edgaholic - Finish a level by getting EDGE time on every edge climbed
Spoiler:
Best done on level 1 - "1st contact"

Pickup - Get picked up by a platform while hanging on an edge
Should unlock soon enough during your playthrough

Hitchhike - Get 30 seconds of EDGE time on a moving platform
Self explanatory

Daredevil - Get more than 0.5 seconds of EDGE time on the active side of a bumper
Piece of cake. Can be done on many levels. Just time your moves carefully

Dropoff - Get dropped off by a platform and continue hanging on the wall
Same as Pickup - just play the game long enough.

Roundtrip - Get picked up by a platform and then dropped of by a platform
Ditto.

Subway - Ride the complete subway track
Spoiler:
Unlocked in level 10 - "metro" by riding to the last station and then back via the other track. Quite easy

Turnstile jumper - Evade the subway fare
Spoiler:
Unlocked in level 10 - "metro" by bypassing the button switches in the beginning of the level

Jumper - Jump on top of a car in "peripherique"
Spoiler:
Level 14 - "peripherique" at the very beginning

Backpack - Get at least 8 seconds of EDGE time on the human figure in "edge time"
Spoiler:
Level 18 - "edge time". Quickly hit the final button in the level, then drop off the block and continue edging on it before the figure reaches the exit point. VIDEO (Thx to ASecondOpinion)

Tumbling travels - Grab the prism after the wall sequence in "higher" as mini cube
Spoiler:
Level 24 - "higher". After changing into a mini cube simply backtrack past the moving platforms and grab the prism. Arches are your friend

Perfect landing - Fall directly on the exit square in "star castle"
Spoiler:
Just time your jump and if you fail fall off the edge and try again. It's on level 31 - "star castle"

Invader - Get on top of an invader
Spoiler:
Just climb one of the invaders on level 40 - "cube invaders"

An enemy of my enemy - Cross the block-enemy section in "cube invaders" without activating any of the three bridges
Spoiler:
Level 40 - "cube invaders". Let the "enemy" carry you over to the other side

Nailed it - Hit the switch halfway through "hammer" before it disappears
Spoiler:
Bonus level 2 - "hammer". Rush through the level as fast as you can, until you hit around about the half way point (This bit being cubes of 3x3x3, with a hole in the middle). One of these cubes has a button that disappears if you take too long to get there. You need to hit that button. VIDEO (Thx to ASecondOpinion)

Monkey - Unleash the totem monkey head in "babylonian"
Spoiler:
Bonus level 4 - "babylonian" You have to press the tiles at the top of the structure in the right order so they don't play a noise when going over them. The tile at the bottom left of the screen counts as the first one. This involves skipping over a few tiles and entering the colored finish zone. If you make a mistake you can simply go back to the first tile at the bottom left to reset everything. (Thx to Perducci)
This is the actual order - N marks north.
Code:
___________________
| 4 | | N | | |
|___|___|___|___|___|
| 2 | o | o | o | |
|___|___|___|___|___|
| 5 | o | X | o | |
|___|___|___|___|___|
| 3 | o | o | o | |
|___|___|___|___|___|
| 1 | | | | |
|___|___|___|___|___|

Mythical bird - Summon the mythical bird in "mystic"
Spoiler:
Bonus level 9 - "mystic". At the part near the end where there are three blocks moving in a row you must get up on the ledge above them. This involves using the second block and climbing up the gap at the right moment. If done correctly that block will lift you up after a second, and all the pieces of the bird will fly into place at the end of the level. (Thx to Perducci)

Shuffled - Get shuffled in "magic"
Spoiler:
Bonus level 7 - "magic". Fall into one of the holes you cannot escape from

Chicken - Find Toki Tori
Spoiler:
Level 24 - "higher". Take a look around the exit square

Quick response - Find the QR code
Spoiler:
It's in the first level. Hit the hidden switch at the start of the level. Then once you make it to the end, circle around the colored blocks clockwise a few times. If you do this then the blocks will start to rearrange. Climb up these moving blocks. Then make your way to one of the ledges sticking out at the top right of the map. Stand there and the QR is revealed! (Thx to Perducci)

Balance - Balance on the floor for at least 5 seconds
Self-explanatory

Compact - Get crushed 10 times
Just play the game long enough and you'll get it

Long Fall - Fall more than 16 meters and survive
Spoiler:
Try the last section in level 14 - "peripherique"

Pinhole - Finish a level in mini form
Spoiler:
Level 25 - "squadron" seems optimal for this achievement. It's easy to figure out once you reach the later portion of this level

Snowblind - Find the six hidden switches in the snow-covered levels in December
Spoiler:
http://forums.steampowered.com/forum....php?t=2293973