A good site documenting approximations of irrationals I'm thinking of Sloane here but I believe that only takes sequences/series into account.
Basically I've derived an interesting, appealing formula for e and want to know if it's already been discovered.

We want to show equality 
$$ e={2 \over 3} \left( {1 \over 1!} +{1 +2\over 2!} + {1 +2+3\over 3!} + {1 +2+3+4\over 4!} + ... \right) $$ 
Let's define a more general expression $f$ using the indeterminate $x$ at each summand inside the parenthese and derive the equality by finally inserting $x=1$:
$$ \begin{array}{}
f(x)&= {1 \over 1!} +{3\over 2!}x + {6\over 3!}x^2 + {10\over 4!}x^3 + ...  \\
&={1 \over 2}\left( {2 \over 1!} +{6\over 2!}x + {12\over 3!}x^2 + {20\over 4!}x^3 + ... \right) \qquad \text{//cancel biggest factors}\\
&={1 \over 2}\left( {2 \over 0!} +{3\over 1!}x + {4\over 2!}x^2 + {5\over 3!}x^3 + ... \right) \qquad \text{//separate in two series} \\
&={1 \over 2}\left( {1 \over 0!} +{2\over 1!}x + {3\over 2!}x^2 + {4\over 3!}x^3 + ... \right) + {1 \over 2}\left( {1 \over 0!} +{1\over 1!}x + {1\over 2!}x^2 + {1\over 3!}x^3 + ... \right) \\
&={1 \over 2}\left( {1 \over 0!}x +{1\over 1!}x^2 + {1\over 2!}x^3 + {1\over 3!}x^4 + ... \right)'+ {1 \over 2}e^x \qquad \text{//note the first derivative!}\\ 
&={1 \over 2}\left( x e^x \right)'+ {1 \over 2}e^x\\ 
&={1 \over 2}\left( 1+ x  \right)e^x+ {1 \over 2}e^x\\
&={1 \over 2}(2+x)e^x\\ \end{array}
$$
and if we insert $1$ for $x$ in $f(x)$ we get equality
$$ e = {2\over 3} f(1) ={2\over 3} \left( {1 \over 2} (2+1) \right)e^1=e  $$
