Fifty Shades of Grey

Mathematica, 30 bytes

Here is another Mathematica approach:

ArrayPlot[#+5#2&~Array~{5,10}]

or

ArrayPlot[5#+#2&~Array~{10,5}]

The first one simply creates an array

{{6, 11, 16, 21, 26, 31, 36, 41, 46, 51}, 
 {7, 12, 17, 22, 27, 32, 37, 42, 47, 52}, 
 {8, 13, 18, 23, 28, 33, 38, 43, 48, 53}, 
 {9, 14, 19, 24, 29, 34, 39, 44, 49, 54}, 
 {10, 15, 20, 25, 30, 35, 40, 45, 50, 55}}

and the second one

{{6, 7, 8, 9, 10},
 {11, 12, 13, 14, 15},
 {16, 17, 18, 19, 20},
 {21, 22, 23, 24, 25},
 {26, 27, 28, 29, 30},
 {31, 32, 33, 34, 35},
 {36, 37, 38, 39, 40},
 {41, 42, 43, 44, 45},
 {46, 47, 48, 49, 50},
 {51, 52, 53, 54, 55}}

Then, ArrayPlot plots them as a grid and, by default, uses greyscale to visualise the values.

CJam - 23 (no actual graphical output)

Since CJam can't draw on the screen (yet?), I wrote a program that outputs an image in plain pgm format.
Save the output to a file called 50.pgm and open with an image viewer/editor.

"P2"1e3K51_,1>K*$K*~]N*

Try it online

The result looks like this:

Explanation:

"P2"     push "P2" (netpbm magic number)
1e3K     push 1000 and K=20 (image size)
51       push 51 (value of white)
_,       duplicate 51 and convert to an array [0 1 2 ... 50]
1>       remove 0 (slice from 1)
K*       repeat the array 20 times
$        sort, to get [(20 1's) (20 2's) ... (20 50's)]
K*       repeat the array 20 times
~        dump the elements on the stack
]        put everything from the stack in an array
N*       join with newlines

Mathematica 72 61 59 43 35 34 bytes

Current version (34 bytes)

GrayLevel[#/50]~Style~50 &~Array~50    

---

Earlier version (59 bytes), thanks to Martin Büttner.

Graphics[{GrayLevel[#/50],Cuboid@{#~Mod~9,#/9}}&/@Range@50]

Blue borders added to highlight position of squares.

Graphics[{EdgeForm[Blue], GrayLevel[#/50], Cuboid@{#~Mod~9, #/9}} & /@Range@50]

Number of squares:

    Length[%[[1]]]

50

---

First attempt (72 bytes)

If the squares can overlap. As Zgarb notes, there is a remote possibility that one square would hide another.

  Graphics@Table[{GrayLevel@k,Cuboid[{0,15}~RandomReal~2]},{k,.4,.89,.01}]