Rupert is the Ximian mascot. Because of how adorable Rupert is, there has become quite a cult following of the monkey. It's presumed that Rupert has also helped to spread the Ximian name.
Rupert comes in many forms. The most enjoyable of which is a 6 inch stuffed animal doll (https://store.ximian.com/images/store/merchandise/monkey-128.png). His nose is shaped like a V with black string and has two plastic eyes situated above it. His shirt is black with the yellow Ximian logo, also called the Monkey Button. The Monkey Button is a circle with a running Rupert inside. Under the logo is 'www.ximian.com'. The Rupert doll is a great gift to people, even if they don't know who Ximian is.
The Rupert logo has gone on to create some enjoyable images and references. Employees at Ximian often refer to themselves as "Monkeys". In the free time, people have also created battle scenes with Ruperts and Nerf toys.
A few stories also exist about seeing Ruperts and the monkey button around. I saw a teenager wearing a t-shirt in Boston on the street. A co-worker also saw a homeless guy wearing a Ximian shirt with the monkey button in Montreal.