qmol is a simple molecular weight calculator, available for Linux and Windows, derived from Tomislav Gountchev's KMol. You can enter a formula, (e.g. CH3(CH2)2OH) and get its molecular weight (60.0959 g/mol) and its elemental composition (C3H8O: C 59.96; H 13.42; O 26.62 %).
See the features list below for more details or consult the wiki page: https://sourceforge.net/p/qmol/wiki/Home/.
For Windows binary and source code see the files section: https://sourceforge.net/projects/qmol/files/
Binary packages (rpm/deb) for
openSUSE 11.4, 13.1-13.2, Ubuntu 12.04, 14.04-15.04, Fedora 20-21, RHEL 7, Scientific Linux 7, and Debian 7.0 & 8.0 can be found at: http://download.opensuse.org/repositories/home:/lineinthesand/
- Pre-defined groups (alkyls/aryls, amino acids)
- User-defined groups
- Protein/oligopeptide calculation mode
- One-letter amino acids parsing in square brackets 
- History of recently calculated formulas
- Configurable output
- Command line calculation
qmol provides an important yet basic functionality that has so far been missing, or at least not comfortably accessible, on linux -- a working and uncluttered molecular weight calculator. Most existing calculators exist as part of larger projects (e.g. mass spectrometric tools), or won't allow abbreviations, lack a gui, or simply won't compile on a modern system (e.g. kmol, the precursor to qmol) -- and even the debian repos, which are presumably the largest repositories among all the linux distributions, do not contain a molecular weight calculator. Features that make qmol stand out are: * the presence of BOTH a gui AND the ability to run calculations from the command line * the ability to easily define abbreviations for chemical groups * a history of recently computed molecular weights/formulae * the computation of % elemental composition Note that the provided .deb package for debian 6.0 won't work on Debian testing/wheezy (i.e. 7) and presumably not on Debian Sid either. It is fairly straightforward to compile though. Instructions for how to compile for Debian Testing can be found online by googling for "qmol compile debian" (you're not allowed to post URLs in reviews, hence the google instructions)