Generally, there are two situations where you want an older version of Glyphs: if you need to support older hardware (read PowerPC processors) and if something that used to work is broken in the latest version and there is no bugfix yet.
You can use the
glyphsapp.com/updates/GlyphsX.X.X.zip scheme to download any version. The
X.X.X part represents the version number. So if you want to download, say, version 2.2.2, you’d type glyphsapp.com/updates/Glyphs2.2.2.zip. It’s that easy.
You can always download the latest stable version here.
Downloading Older Betas
If you want to download a beta version, e.g. the beta 844 of version 2.3 , you will have to type, glyphsapp.com/updates/Glyphs2.3b-844.zip. At the moment,
Glyphs2.3b-838 is the latest available version.
The change log is incorporated into the application. Just go to Help > Change Log, and Glyphs will open a window containing the version history for you.
Glyphs for PowerPC Macs
You will get the latest Universal binary, i.e. the latest version that also supports older Macs with a G4 or G5 processor, by using this link: glyphsapp.com/updates/latestPPC.php. For now, the latest PPC enabled version is version 1.2.6.
Update 2013-07-18: Added note about the change log in newer versions.
Update 2016-02-16: Updated for Glyphs 2 and added beta downloads (thanks oneweioranother!)