Curve Fitting and Function
Minimization  Robust nonlinear least squares curve fitting
(MPFIT), statistical tests, specialized peak fitting functions, and
nonlinear optimization (TNMIN) 
Read and Write SAVE
Files  Library to read, write and interrogate IDL SAVE files
(CMSVLIB); read and translate compiled IDL procedures; documentation
of IDL SAVE file format.

Graphics Programs 
Display an image using a PLOTlike interface;
display colorful bar charts;
plot three dimensional data as a cube;
plot fixed or variablewidth histograms

Mathematics 
Adaptive numerical integration (Quadpack);
Chebyshev approximation and interpolation;
(Modified) Cholesky factorization of a symmetric matrix;
QR matrix decomposition;
Manipulate quaternions

Printing Utilities 
Oneclick printing (Unix);
configure Postscript layout;
enhanced XWINDOW

Introspection 
Generate a type description or "help" description of an IDL value;
compute type information for tags in a structure;
unofficial documentation for ROUTINE_NAMES

Ephemeris & Timing 
Interpolate the positions and motions of planets (JPL ephemerides);
read and interpolate geopotentials;
calculate relativistic clock corrections in the solar system;
compute relativistic velocity additions;
compute high precision earth orientation angles;
compute leap seconds.

Array / Set Operations 
Hash tables;
Manipulate 1D Intervals (GTIs);
compute the fast product of a large array;
efficiently insert / delete elements in an array;
perform set operations on numbers and strings;
generic replacement for IDL's REPLICATE;
apply generic function to array

Input / Output 
Capture a transcript of console output;
read ASCII tables quickly;
construct path names;
read and seek on pipes/URLs like normal files (Unix)

Miscellaneous 
Unique identifiers;
Command line debugging made easy;
compute and convert between time representations;
maintain a oneline status line (Unix/ANSI);
improved image interpolation
