Potential advertisers always ask me how much traffic my website has, the number of unique visitors, etc. I have all these stats but I would like more transparency (otherwise how do they know I haven't enhanced the stats?).
Quantcast is a great solution, but if you want more control over your data, I think Piwik would offer you a great solution (ok I'm a bit biased, being part of their team, but it is awesome). You can create a profile for your advertisers and they can check the stats themselves, or you can create an iframe with the specific stats, on a secure page which they can check at their leisure. You can check out a demo of Piwik at http://demo.piwik.org
Most of the online advertisers I have used have used Alexa ranking as a rough estimate how much traffic my site receives.
Google, Alexa, Quantcast and all other stat based programs can be faked using plenty of proxies. The real test is checking your rankings and ensuring that your being found on certain 'KEYWORD PHASES' they will want to be targeting. Stats are easy to fake, Google Results are not.
Why not add them as a user to that Analytics profile and let them see your stats for themselves after signing an NDA. I wouldn't believe any other service that's estimating your stats without access to an account.
If i'm going to spend money advertising on a website I'm not going to believe any site estimating your traffic other than one which has a tracking snippet installed and one which has been there for at the least 6 months. I would want to see how much organic traffic, and referral traffic you get as well as page views.
If you inflated your own traffic it would probably show a big spike at some point in direct or strange referral traffic.