FieldTrip is the MATLAB software toolbox for MEG and EEG analysis that is being developed by a team of researchers at the Donders Institute for Brain, Cognition and Behaviour in Nijmegen, the Netherlands in close collaboration with collaborating institutes.
FieldTrip offers advanced analysis methods of MEG, EEG, and invasive electrophysiological data, such as time-frequency analysis, source reconstruction using dipoles, distributed sources and beamformers and non-parametric statistical testing. It supports the data formats of all major MEG systems (CTF, Elekta/Neuromag, 4D, Yokogawa) and of most popular EEG systems, and new formats can be added easily. FieldTrip contains high-level functions that you can use to construct your own analysis protocols in MATLAB. Furthermore, it easily allows developers to incorporate low-level algorithms for new EEG/MEG analysis methods.
Furthermore, it offers a frequently updated website  with tutorials, example data and other documentation. There is an active community centered around the email discussion list .
FieldTrip is the mostly used software for MEG data analysis and developed in close collaboration with related open source projects, such as EEGLAB, SPM, BrainStorm and MNE.
Please cite the FieldTrip reference paper  when you have used FieldTrip for data analysis in your study.
Fieldtrip is excellent, it has great documentation for what is a very specialised tool, and is very actively maintained and easy to contribute to. I use it for LFP and neurophysiological spike analysis, which are not the main aims of Fieldtrip but it performs these functions well.
Suggestions and questions about the FieldTrip toolbox should be posted on the email discussion list (see http://www.fieldtriptoolbox.org/discussion_list), on bugzilla (see http://www.fieldtriptoolbox.org/bugzilla) or through the github page (see http://www.fieldtriptoolbox.org/development/git).
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