In Google Contacts/Gmail you can assign a contact to a group (or more than one group). I use this so that on my Android phone I can filter the list of contacts down to just work colleagues, just family, just people I know through a hobby or sport, etc.
I'm pretty disciplined about always adding people into a group when I add them through the web interface at a PC, but I often forget if someone gives me their phone number when I'm out-and-about, for instance.
I can filter the list of contacts in the web interface by clicking the group name to see people in that group, but there doesn't seem to be any way to filter to just see people who aren't in any groups?
Mine is a Google Apps account, in case it matters, but I'd be happy with an answer that only works on the main Gmail on the basis that these features come to Apps accounts eventually.