As any release it includes numerous bugfixes, cool features and performance improvements - in 387 commits since 0.23.1.

0.24.0 was a preview release so this is the canonical realease following 0.23.1.

Exciting Changes

Added ThinLTO support for LLVM 4.0 and above, which speeds up release builds. It can be activated using the compiler option --lto=thin.
(breaking-change) SecureRandom is renamed to Random::Secure and SecureRandom.uuid is replaced by a new UUID type, created using UUID.random.
BigDecimal was added, it offers arbitrary precision, exact decimal numbers. Supplementing BigInt and BigFloat with all three being included using require "big".
Set instances can now be used as condition for case, matching when the case variable is included in the set.
(breaking-change) Time::Format more robustly supports parsing sub-second precision.
Tempfile now supports custom extensions.
The docs generator outputs documentation as JSON which can be easily parsed and used for external applications. The output directory of the docs generator can be configured using --output flag.
Garbage collection can be disabled using compiler flag -Dgc_none.
The default hash function uses an algorithm based on FunnyHash which is resiliant against DoS attacks.
(breaking-change) HTML.escape only escapes HTML special characters &<>"'. HTML.escape recognizes all HTML5 named entities.
(breaking-change) Multiple assignment can no longer be used to create an array (a = 1, 2, 3).
(breaking-change) skip macro method was renamed to skip_file.
rescue, ensure and else blocks can be used directly in do ...end blocks.

%w(foo bar).each do |x|
  raise x
rescue e
  p e
end

fun names and lib function calls can now start with uppercase.
YAML 1.1 core schema is now fully supported.
Time and Time::Span have nanosecond precision.
Time.monotonic and Time.measure { ... } allow reading and measuring time using a monotonic clock not influences by time zones, leap seconds and time corrections.
Crystal.main was added to allow more easily redefining the main of a program.
Named tuples can be merged using NamedTuple#merge(other : NamedTuple).
YAML and JSON mappings support presence: true option.
Dir has two new class methods: children and each_child { ... } which include all entries except . and ...

Other Breaking Changes

(breaking-change) HTTP::Client#post_form is now HTTP::Client.post(form: ...)
(breaking-change) Array#reject!, Array#compact! and Array#select! now return self
(breaking-change) Remove the possibility to require big_int, big_float or big_rational individually: use require \"big\" instead
(breaking-change) Spec: remove expect_raises without type argument
(breaking-change) IO is now a class, no longer a module
(breaking-change) Time constructors now have nanosecond and kind as named argument
(breaking-change) Removed XML.escape. Use HTML.escape instead
(breaking-change) Removed macro def
(breaking-change) To define a custom hash method you must now define hash(hasher)
(breaking-change) Flate::Reader.new(&block) and Flate::Writer.new(&block) now use the name open
(breaking-change) Use an Enum for Process stdio redirections
(breaking-change) Remove ‘$0’ special syntax
(breaking-change) StaticArray#map and Slice#map now return their same type instead of Array
(breaking-change) Tuple#map_with_index now returns a Tuple.

Thanks to everyone who supported this release through contributions, reviews and suggestions.