Andy Gregorowicz's Ruby Style Guide (mostly copied from others)

Formatting:

Use ASCII (or UTF-8, if you have to).
Use 2 space indent, no tabs.
Use Unix-style line endings.
Use spaces around operators, after commas, colons and semicolons, before "{" "}".
No spaces after "(", "[" and before "]", ")".
Use two spaces before statement modifiers (postfix if/unless/while/until/rescue).
Indent when as deep as case.
Use an empty line before the return value of a method (unless it only has one line), and an empty line between defs.
Use RDoc and its conventions for API documentation. Don't put an empty line between the comment block and the def.
Use empty lines to break up a long method into logical paragraphs.
Keep lines fewer than 80 characters or have good reason not to.
Avoid trailing whitespace.

Use def with parentheses when there are arguments.
Never use for, unless you exactly know why.
Never use then.
Try to use "'" over """ and "%q()" over "%Q()"
Use when x; ... for one-line cases.
Use &&/|| for boolean expressions, and/or for control flow. (Rule of thumb: If you have to use outer parentheses, you are using the wrong operators.)
Avoid use of ?:
Methods should have parenthesized options, unless they are called as DSL components (e.g. Witness.new(:statement => 'Lots of important stuff', :format => :plaintext) but read_a :book, :when_it_is_interesting)
Prefer do...end over {...}. Curly braces are acceptable for one line blocks and code chaining.
Avoid return where not required.
Avoid line continuation () where not required.
Using the return value of = is okay:

if v = array.grep(/foo/) ...
Use ||= freely. It can be used to initialize a varible if it is nil.
Use non-OO regexps (they won't make the code better). Freely use =~~, $0-9, $~~, $` and $' when needed.

Use snake_case for methods.
Use CamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.)
Use SCREAMING_SNAKE_CASE for other constants.
The length of an identifier determines its scope. Use one-letter variables for short block/method parameters, according to this scheme:

a,b,c: any object d: directory names e: elements of an Enumerable ex: rescued exceptions f: files and file names i,j: indexes k: the key part of a hash entry m: methods o: any object r: return values of short methods s: strings v: any value v: the value part of a hash entry x,y,z: numbers

And in general, the first letter of the class name if all objects are of that type.
When using inject with short blocks, name the arguments |a, e| (mnemonic: accumulator, element)
When defining binary operators, name the argument "other".
Prefer map over collect, find over detect, find_all over select, size over length.

Avoid hashes-as-optional-parameters. Does the method do too much?
Avoid long methods.
Avoid long parameter lists.
Use def self.method to define class methods. Avoid class << self blocks.
Add "global" methods to Kernel (if you have to) and make them private.
Avoid alias when alias_method will do.
Use OptionParser for parsing complex command line options and ruby -s for trivial command line options.
Write for 1.8, but avoid doing things you know that will break in 1.9.
Avoid needless metaprogramming.

Code in a functional way, avoid mutation when it makes sense.
Do not mutate arguments unless that is the purpose of the method.
Do not mess around in core classes when writing libraries.
Do not program defensively. (See http://www.erlang.se/doc/programming_rules.shtml#HDR11.)
Keep the code simple.
Read other style guides and apply the parts that don't dissent with this list.
Be consistent.
Use common sense.