Ooh boy, it really depends.

On the flats, if I'm feeling good I can sustain about 16-17' mph.
On the flats, if I'm feeling good and there's a tailwind or a very slight decline, I might hit 18-20 mph for a while.
Going up moderate hills 8-9 mph.
Going up very steep hills 4-5 mph - barely keeping the bike vertical.

The computer average at the end of almost any mixed flats/hills/stoplights ride usually says 15 mph. 14mph total average if I'm out of shape. On centuries and longer rides, it starts to drop down to 13 especially if I am having nutrition issues.

This thread is very reassuring, I'd always considered myself to be a somewhat pokey rider. Though, now that I think about it... until this past year when I finally upgraded to clipless pedals and trained for and rode the Seattle to Portland, I was in the 12-13 range for years. I guess my self-perception just hasn't quite caught up yet.