How can I plot a Skew-T log-P diagram?
Skew-T Log-P diagrams can be plotted in most mathmatics software.  I have seen examples in Matlab (and by proxy Octave), GrADS, NCL, IDL and I have my own code in Python that generates Skew-T plots.  There are also some programs such as NSHARP and BUFKIT that are more full featured.  You can also find websites that plot forecast soundings such as twisterdata and real data soundings.

The major variances arise from the specific use case of whoever wrote the code you are using.  The most basic code will generally be able to tell you the lifting condensation level (LCL), level of free convection (LFC), equilibrium level (EL) and perhaps the 500 mb lifted index, all for a surface parcel.

The more advanced code will generally be able to calculate the above quantities not just for surface parcels, but also for the mixed layer and be able to find the most unstable parcel.  Convection oriented code will give you CAPE, storm tops and other severe weather parameters and will also tend to be able to analyze the hodograph.  BUFKIT can do analysis to identify snow growth levels and some other winter weather related quantities.

One of the subtle differences in skew-T plotting software is how they handle the moist adiabats.  This can be modeled as a fully reversible process or a pseudo reversible process and this produces subtle temperature differences aloft on the adiabat.  There can also be differences in whether the moist adiabat only considers liquid water and vapor, or if ice process are included in the latent heat release.  

Here is an example of what my Python code produces:  



Var          SFC            ML              MU
CAPE  J/kg   1877           1877            2415
CIN   J/kg   46.9           46.9            0.4
LI_max       -9.5           -9.5            -11.3
LI_500       -7.3           -7.3            -8.4
LI_30        -4.8           -4.8            -6.4
TOPS  m      14800          14800           15400
LCL   mb (m) 892 (1000)     892 (1000)      862 (1292)
LFC   mb (m) 831 (1604)     831 (1604)      857 (1343)
PRS   mb     1000           1000            871