I understand the nagging issue. I also feel it that way, but not for Wikipedia or other sites devoid of advertising. Sensitivity may vary though. Getting a short text explaining the NG philosophy, and asking for a contribution at sign in would not be interpreted as nagging, I think. Same for a yearly message to all visitors, around the anniversary of the site, for exemple.
NG is an exception. I think it is worth reminding visitors why it is so and how it can survive the current extinction phase of high quality open access online communities.