Android 4.0 ("Ice Cream Sandwich") is the latest version to show up on smartphones and tablets, but many devices are still being sold running an older version of Android, the most common being Android 2.3 ("Gingerbread").
However, a community of mobile hardware modders, CyanogenMod, has been working to port ICS to older-model Android devices. These volunteer developers are best known for bringing Gingerbread to such devices, naming their port CyanogenMod 7.
CyanogenMod 9, currently being developed, will enable the porting of ICS to a number of specific devices. Here are some of those. (Note: When you upgrade a device with firmware not officially supported by its maker, you are responsible for damage that can occur to your device if you don't follow carefully the installation instructions.)