+1 live 'any language' syntax check and basic errors would be great actually. For example for Python, running pep8 or pylint and put a sublte bubble with the number of warning (on which you can click to get more infos).
This could be done similar to Build menu. It's simple to create a plugin to run a syntax checker command depending on the file extension/mime (example: php -l) and say ok or not. It might be possible to do it before saving (avoiding you to save a file without being ok).
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