Being a part of a writing group is absolutely necessary when writing anything that people will read. If you sit alone at your computer, churning out self-published novels without the input and critique of others you are only reaching half of your potential as a writer. Writer's groups pull this potential out of you and help you feed it back into your text.
If you live out in the middle of nowhere like me or your city does not have a writer's group, then you need to join a writing forum. Below is a list of forums that are at least a start for writers looking to get critique of their work.
1. Creative Writing Forums - This forum is a straight forward forum that helps writers hone their craft in poetry and short stories. If writing a novel, one should upload sections or chapters to this forum in order for the other writers to give a good critique. Writers can also discuss difficult plot points, characterization or any other writing problem that may arise.
2. Writer's Beat - This forum is a little different in that they are a place where writers can probably find a published author to help critique their work. It is also a place where (with enough digging) one can find someone who will critique an entire novel or at least point writers in the right direction.
3. Absolute Write - This forum is more of a mom and pop independent setup that is more of the best of both of #1 and #2 on this list. It has a "water cooler" where writers can discuss all kinds of industry topics but it also has a really good section on disreputable publishers, scams and other things that will send the self-published author into a tailspin.
Some things to remember:
1. Expect Critique - People who frequent forums, especially writing forums, are brutally honest. If you do not handle criticism well, then you should not frequent a writing forum (and should possibly consider doing something else besides writing).
2. Beware of Trolls - Sometimes bad writers will troll these forums for ideas or plot lines. If you haven't copyrighted your text, then you need to get the paperwork filled out and do that before posting anything about your novel. Somebody might steal it, then say that you copied their idea. It's not a perfect world.
3. Be Kind, But Honest - If you critique people's work, please understand that they are people with feelings. Critique the work only and not the person. Use positive adjectives when critiquing a work, even if it is sub-par. Every time you find fault, give positive suggestions for remedying the fault.