• For general purpose spell-checking, there are quite a few popular alternatives
  1. ispell and friends: Built into emacs & typically called with ispell-buffer. Checks spelling only on demand.
  2. flyspell-mode: Also built-in and provides on-the-fly spell checking and highlights mistakes.
  3. speck-mode: Available from MELPA, it checks the spelling of the word once you pause after typing. Also has a few distinguishing features from flyspell mode like able to use multiple dictionaries in the same buffer. (Disclaimer : I only used this mode briefly a long time back)

Note that all the above require dedicated spell-checking programs such as aspell or hunspell and appropriate dictionaries. The modes only provide a convenient emacs interface to the command line programs.

• For selective spell checking, you can customize flyspell-mode with the variable flyspell-generic-check-word-predicate by wiring a function that selects which words must be checked. Your listed example is easy though -- Just use flyspell-prog-mode which only checks comments in code buffers.

• Heuristic spell checking: I am not aware of any dedicated grammar parsers for emacs yet. I will mention writegood-mode available in MELPA which highlights weasel words and passive voice in the buffer. It gets you half the way there.

• Anything else: Correcting language in general is a hard problem. That being said, if there exists any external program/script in this area, it is usually easy to make it interface with emacs.

• For general purpose spell-checking, there are quite a few popular alternatives
  1. ispell and friends: Built into emacs & typically called with ispell-buffer. Checks spelling only on demand.
  2. flyspell-mode: Also built-in and provides on-the-fly spell checking and highlights mistakes.
  3. speck-mode: Available from MELPA, it checks the spelling of the word once you pause after typing. Also has a few distinguishing features from flyspell mode like able to use multiple dictionaries in the same buffer. (Disclaimer : I only used this mode briefly a long time back)

Note that all the above require dedicated spell-checking programs such as aspell or hunspell and appropriate dictionaries. The modes only provide a convenient emacs interface to the command line programs.

• For selective spell checking, you can customize flyspell-mode with the variable flyspell-generic-check-word-predicate by wiring a function that selects which words must be checked. Your listed example is easy though -- Just use flyspell-prog-mode which only checks comments in code buffers.