Interval package
The GNU Octave interval package for realvalued interval arithmetic.
The interval package provides data types and operations for verified computing. It can be used to handle uncertainties, estimate arithmetic errors and produce reliable results. Also it can help find guaranteed solutions to numerical problems. Thanks to GNU MPFR many arithmetic operations are provided with best possible accuracy. The implementation of intervals in infsup format is based on interval boundaries represented by binary64 numbers and is standard conforming to the (proposed) standard for interval arithmetic IEEE P1788.
Distribution
 Latest version at Octave Forge (download, function reference, and package documentation)
 MacPorts for Mac OS X
 FreshPorts for FreeBSD
Development status
 Completeness
 All required IEEE 1788 functions are implemented, the standard is currently in recirculation ballot phase and quite stable.
 Planned: more solvers; plotting functions
 Planned: Improvement of the user documentation
 Quality
 Includes tests for all functions, many tests for basic functions
 No known bugs. The package is quite new and still has a small user base, so there might be hidden bugs. Also some advanced functions need more testing.
 Portability
 Runs in GNU Octave 3.8.2 and 4.0 release candidates
 Known to run under GNU/Linux, Microsoft Windows, Mac OS X and FreeBSD
Compatibility
The interval package's main goal is to be compliant with IEEE 1788, so it is compatible with other standardconforming implementations (on the set of operations described by the standard document).
This interval package is not meant to be a replacement for INTLAB and any compatibility with it is pure coincidence. Since both are compatible with GNU Octave, they happen to agree on many function names and programs written for INTLAB may possibly run with this interval package as well. Some fundamental differences that I am currently aware of:
 INTLAB is nonfree software, it grants none of the four essential freedoms of free software
 INTLAB is not conforming to IEEE 1788 and the parsing of intervals from strings uses a different format—especially for the uncertain form
 INTLAB supports intervals with complex numbers and sparse interval matrices, but no empty intervals
 INTLAB uses inferior accuracy for most arithmetic operations, because it focuses on speed
 Basic operations can be found in both packages, but the availability of special functions depends
Code: In GNU Octave the interval package can also be run alongside INTLAB. 
# INTLAB intervals
A1 = infsup (2, 3);
B1 = hull (4, A1);
C1 = midrad (0, 2);
# Interval package intervals
pkg load interval
A2 = infsup (2, 3);
B2 = hull (4, A2);
C2 = midrad (0, 2);
pkg unload interval
# Computation with INTLAB
A1 + B1 * C1
# Computation without INTLAB
A2 + B2 * C2

Similar software
For C++ there is an open source interval library libieeep1788 by Marco Nehmeier (member of IEEE P1788). It aims to be standard compliant with IEEE 1788 and is designed in a modular way, supporting several interval data types and different flavors of interval arithmetic [1]. The GNU Octave interval package shares several unit tests with libieeep1788.
For C++, Pascal and Fortran there is a free interval library XSC. It is not standard compliant with IEEE 1788. Some parts of the GNU Octave interval package have been derived from CXSC.
For MATLAB there is a popular, nonfree interval arithmetic toolbox INTLAB by Siegfried Rump. It had been free of charge for academic use in the past, but no longer is. Its origin dates back to 1999, so it is well tested and comprises a lot of functionality, especially for vector / matrix operations. INTLAB is compatible with GNU Octave since Version 9 [2].