Is there a site to draw a curve and get the equivalent equation?

Your curve appears to be the cubic spline parametrized by $$ p(t) = (1 - t)^{3}\, p_{1} + 3(1 - t)^{2}t\, p_{2} + 3(1 - t)t^{2}\, p_{3} + t^{3}\, p_{4},\quad 0 \leq t \leq 1, $$ with $p_{1} = (0, 1)$, $p_{2} = (\frac{8}{3}, 1)$, $p_{3} = (\frac{4}{3}, 0)$, $p_{4} = (4, 0)$.

Dmitry Baranovskiy's Raphaël JavaScript library has an interactive demo for drawing splines.

Try this:
$y=\frac{1+\cos{\frac{\pi·x}4}}2$
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RODRIGO'S COMMENT: Very good, Senex, thank you. But I like Weaam solution more, because I have more control to adjust it (closer to a sinoid vs. closer to a straight line). – Rodrigo
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EDIT PURSUANT TO RODRIGO'S COMMENT (2016·3·6):
Well, try this then:
C and L are weights to control how much influence the cosine curve and the straight line, respectively, will have in the shape of your desired curve.

examples:
$C=1\;\text{and}\;L=0\;\text{will give you the curve in my original presentation.}$
$C=0\;\text{and}\;L=1\;\text{will give you the pure straight line.}$
$C=1\;\text{and}\;L=1$
$\;\text{will give you a curve in which the two influences are balanced.}$
Other combinations of C and L will yield other results.

$y=\frac{C·\frac12·\left(1+\cos{\frac{\pi·x}4}\right)+L·\frac{4-x}4}{C+L}$