Acorn 4.4 May 26th, 2014.
Acorn 4.3.1 February 24th, 2014.
Acorn 4.2.3 January 4th, 2014.
Acorn 4.2.2 Dec 10, 2013.
Acorn 4.2.1 Dec 8, 2013.
Acorn 4.2 Dec 6, 2013.
New Stuff:
Bug Fixes:
Acorn 4.1.1 September 18th, 2013.
Acorn 4.1 August 12th, 2013.
Acorn 4.0.5 June 20th, 2013.
Acorn 4.0.4 May 21, 2013.
Acorn 4.0.3 May 7, 2013.
Acorn 4.0.2 May 2nd, 2013.
Acorn 4 May 2nd, 2013.

Tools get their own palette now.
It was getting cramped in there, so we broke out the tool icons into their own palette. In addition we updated a bunch of icons and made everything look better in general.

Layer styles and Filters sitting in a tree…
We rolled the layer styles and filters interface into a single non-modal window. This also means that filters are non destructive! You can still chain together filters like before, but now you can undo and redo your adjustments and even save them as part of your Acorn file. In addition to the filter settings being in a non modal window- you can now directly modify filter parameters right on the layer by dragging any little circles that show up when the filter window is present! (And if you'd like the previous filter window back for some reason- we're keeping it around for a little while- just hold down the option and command keys when choosing your filter from the Filters menu.)

The one exception where the filters window isn't modal (and is destructive) is when you are using a filter on a selection.

Curves Adjustments (finally!)
Acorn now has a Curves Adjustment tool, which you can get to from the Image ▸ Curves… menu. To add a new point on a curve, click on the curve and start dragging. To remove a point on a curve, drag it either left or right until it's past the location of the next point on the curve.

Vector tool improvements.

Levels tool improvements.
The Levels window got a complete rewrite, with a gamma option (to control mid-tones) as well as an option to update the histogram live.

Speed improvements.
Compositing, filter previews, and everything in general is faster. Way faster Flood Fill, Magic Wand, and Instant Alpha tools. The Render Clouds filter got a little speed bump as well.

Multiple layer selection.
The layers list now allows you to select multiple layers. This means you can re-order, delete, move, or lock a bunch of layers at once.

Other great things worth mentioning.

Stuff for Scripters:

Acorn 3.5.1 January 22, 2013
Bug Fixes:
Acorn 3.5 October 9, 2012
New Stuff: Changes: New Stuff for Scripters: Lots of Important Fixes:
Acorn 3.3 July 15, 2012
Acorn 3.2.2 May 1, 2012 Acorn 3.2.1 February 16, 2012 Acorn 3.2 December 2, 2011 Acorn 3.1.2 October 11, 2011 Acorn 3.1.1 September 23, 2011 Acorn 3.1 September 19, 2011 Acorn 3.0.3 July 15, 2011 Acorn 3.0.2 May 31, 2011 Acorn 3.0.1 May 5th, 2011 Acorn 3.0 April 12th, 2011 Acorn 2.6.5 Acorn 2.6.4 Feb 4, 2011 Acorn 2.6.3 Jan 10, 2011 Acorn 2.6.2 October 29, 2010 Acorn 2.6.1 October 25, 2010 Acorn 2.6
October 21, 2010 Acorn 2.5.1
September 27, 2010 Acorn 2.5
September 16, 2010 Acorn 2.3.2
August 2, 2010 Acorn 2.3.1
May 17, 2010 Acorn 2.3
April 30, 2010 Acorn 2.2.2 Acorn 2.2.1 Acorn 2.2 Acorn 2.1 Acorn 2.0.3

Acorn 2.0.2

Acorn 2.0.1 Acorn 2.0 release notes:

The Big things: The WHOLE list, including bug fixes: Acorn 1.5.3 release notes: Acorn 1.5.2 release notes: Acorn 1.5.1 release notes:

Acorn 1.5 release notes:

New Features: Changes / Additions: Bug Fixes:
Version 1.2.1 Version 1.2 Version 1.1.1 Version 1.1 Version 1.0.3 Version 1.0.2 Version 1.0.1 Acorn 1.0
September 10, 2007.