Example: I am using adsense on a site

I sign up for
http://www.widgetbucks.com

And at the top of my widget, I put "Suppport My site"

How does that affect the overall of things? Is that allowed or not?

It implicitly conveys the same message as "Click this," so I believe it would be in violation of AdSense ToS. But, hey, who am I to tell you what to do? I wouldn't try it, but it's a risk you can choose to take.

If you present another way to support your site, say by people making a donation, and that's much closer to your "support my site" banner, then you're probably ok. But if the only way a person could show their support is by clicking ads, then don't expect to have an AdSense account for very long.

It depends on the TOS of widgetbucks

Oops, forgot that he was using WidgetBucks. This is an AdSense forum so I didn't pay much attention to the link. Just read the ToS, like always...

I would avoid doing this
You can ad a donation button via PayPal or related payment sites. I don't know if Google checkout has a donation button or not? I used to have a "donate" button on my site set a $1.00 and I did receive some donations, but not much.

don't force your visitors to click your ads it would cause a violation or infraction to your account and it could possible to ban your account as well your site or blogs. adsense must not look an ads...

Anything that could be seen as encouraging visitors to click your links will be seen by Google as breaking their ToS. A PayPal donations button is allowed though, and you should get a few bucks if your site is genuinely useful to people.