Folk who volunteer as beta-readers should absolutely be familiar with all the material detailed in the first group of links, and it wouldn't hurt to have a good, solid grasp on the second.
Grammar and Mechanics
Rules are, of course, made to be broken. However, you have to know what the rules are in the first place.
Things To Look Out For: A short, non-exhaustive list of things that have, at one time or another, annoyed the hell out of petronia and/or dipping_sauce.
all the punc that's fit to ...tuate?: A brief examination, with examples and explanations, of punctuation standards.
Common Errors in English Usage: The list is extensive, nearly to the point of being cumbersome, but if you've ever wondered for even a second if you might be using 'affect' when you really mean 'effect', this is how you check it. Plus, the author's funny, which makes everything even more delightfully readable.
The Tongue Untied: A Guide to grammar, punctuation and style: Detailed discussions of all major aspects of English grammar, starting at the general (e.g., defining a noun) and working down to the specific (e.g., an indirect object). You can even find quizzes and a ten-week curriculum to make you a grammar master!
English Grammar FAQ (as posted to alt.usage.english): A list of real English questions answered by a linguistics professor. The questions deal with both spoken and written English, though the latter are obviously more relevant, as it's probably not going to matter in text whether or not 'boy' and 'buoy' sound the same.
Guide to Grammar and Writing: Though these guides are geared more for scholarly writing and less for fiction, they've also got many good topics with examples -- and they, too, will quiz you if you want to make sure you're getting it!
Reasoning With Vampires: Regardless of what you think of the Twilight novels, it's plain that the writing in the books ... could be better, to put it mildly. This tumblr seeks to teach about bad writing by example, and at the very least, with any luck, will make you paranoid about comma usage.
hinotori_writes' Shousetsu Bang*Bang Word Counter: Not only does this script count words (useful), it also does a tally of high-frequency words you might not want to be quite so high-frequency. This is also particularly important because a noticeably high level of words ending with -ly basically indicates generally sloppy fiction writing.
Content and Execution
While there is often nothing more tedious than writing about writing, these are of actual, practical use.
Turkey City Lexicon – A Primer for SF Workshops: Though this is SF-specific, the pitfalls it enumerates can be found far beyond the boundaries of science fiction.
How to Write a Sex Scene: Some good tips for how to work a m/m scene into a larger story plot, concerned mainly with the literary merits of the exercise.
Minotaur's Sex Tips for Slash Writers: The nitty-gritty of getting Tab A into Slot B, put together by an Actual Gay Man. If you've got a specific point of curiosity, you might try the very related MQA, a searchable list of questions with detailed answers.
Kurt Vonnegut's eight rules for writing fiction: Perhaps the truest true thing ever.
Big thanks to drmoonpants for using her library-fu to help expand the first section!
See also: Writing Aids and Standards version 1, May 2007.