The Semicolon
It's not a full stop, it's not a comma, so what the hell is it?

The Colon
No we're not talking about that handy little bit of the intestine but those two little dots doing a balancing act.

Bear/Bare
If you're a little confused about whether you've just seen a nudist or a grissly, read on.

Pronoun/Antecedent Agreement
Is anybody a his, her or their?

Getting Down to Cases
Is it me or is it I?

Who/Whom
If you don't have a butler to tell you the difference, let Sister Mary Grammatica.

When choosing an online dictionary be sure to find one that caters to your specific version of the English language or a version that corresponds to how you've been taught the language, if it is not your first.

freesearch
English dictionary - UK

Dictionary.com
English dictionary - US (UK)

Ask Oxford
English dictionary - UK

Ask Oxford
English dictionary - US

Chambers
English dictionary - UK (US)

Merriam-Webster Online
English dictionary - US

YourDictionary.com
English dictionary - US

AlphaDictionary
English dictionary

HyperDictionary Online Dictionary
English (US), computer, medical and dream dictionary

Dictionary of English slang
English slang and colloquialisms - UK

On-Line Medical Dictionary
Medical dictionary

The Elements of Style by William Strunk
An entire book on grammar on-line.

Online Etymology Dictionary
Word origins and meanings.

Common Errors in English
Advanced nitpicking.

Chicago Manual of Style Online
A usage guide to American style - US

Tameri guide for Writers
Word usage and abuses - US

Style guide wiki
Style guide - UK, Canada and US

Chicago Manual of Style
Style guide - US

RhymeZone rhyming dictionary & thesaurus
Rhyming dictionary - US

All of the titles below should be available from Amazon and other book sellers.

Eats, Shoots & Leaves
By Lynne Truss
An amusing and informative look at English punctuation and grammar.

Lapsing into a Comma
By Bill Walsh
A curmudgeon's guide to grammar.

Usage and Abusage
By Eric Partridge
Declaring war on bad grammar, woolliness of expression and poor choice of vocabulary.