Bessel functions aren't supported (and even Excel supports them!)
Try
BesselJ[1,2]//N
On mathematica I get
$ /Applications/Mathematica.app/Contents/MacOS/MathKernel
Mathematica 8.0 for Mac OS X x86 (64-bit)
Copyright 1988-2010 Wolfram Research, Inc.
In[1]:= BesselJ[1,2]//N
Out[1]= 0.576725
In excel, `BESSELJ(2,1)` is the same to 5 significant figures.
"Developers Wanted" is on the first page. Being on open source project, you'll probably see a lot of organic growth and people scratching their own itches.
Try
On mathematica I get In excel, `BESSELJ(2,1)` is the same to 5 significant figures.