I can't seem to find a list of old versions.
From Eclipse you can follow the normal Force.com IDE Plugin instructions, except you can choose to not Show only the latest versions of available software.
Help | Install New Software...
Click Add.. button to add a new site.
Call It Force.com IDE and give it the value of http://www.adnsandbox.com/tools/ide/install/ and click OK.
Untick the Show only the latest versions of available software
Expand the Force.com IDE node and you'll see that there are the previous versions
Alternatively you can go directly to the previous version (26) by using the site URL of http://www.adnsandbox.com/tools/ide/previous.