Emulating the XO
In the past, OLPC produced alternative software images which could be run in emulators such as QEMU and VMware. This meant that development and testing could happen to a limited extent without requiring an XO laptop.
In 2013, please see Tom Gilliard's many virtual machine images ready-to-go.
Note, XOs are readily available to genuine volunteer contributors through the Contributors program.
Sugar, the unique user interface of the XO laptops, is also distributed as a generic software project and can be developed and tested on "regular" computers as well as XOs. See Sugar Labs Downloads for the full range of options, such as Sugar on a Stick.
For emulation of extremely old builds, you may be able to come across "ext3 images" on some corners of http://download.laptop.org and http://xs-dev.laptop.org/~cscott/olpc/streams/ which can be loaded into emulators. You are unlikely to find support for them!