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Based on the last several Google Penguin updates, if your site was going to be harmed or your rankings were going to go down, they probably would have gone down already. That said, that doesn't mean that you could still be penalized. If there are sitewide badge links, they need to look natural. I wouldn't use any specific keyword anchor text, I would use your website name or brand or URL as the anchor text in the link. Don't make it look like you're trying to "game" Google by having too much exact match anchor text. If these links are only a small percentage of the overall links that you have pointing to your site, then there won't be a problem. But if you have only 5 percent like you're saying then it should be fine.

I would place the widget on my site and and anyone who finds it useful can like it, tweet it, link to it. Make these widgets awesome and make your website the "go to place" for these calculations.

Hey there, I wouldn't worry too much about this. I think Google would be smart enough to know that in the context of your website this is valuable content. However, be sure to back this up with good unique content throughout your website. Perhaps for each currency conversion page you could have a passage of text such as... "The US dollar has traditionally been stronger than the GBP, however in recent years we have seen that this difference is contracting. Perhaps one of the reasons for this is blah blah blah blah." That would be a nice lead in to an article about that rate, which in turn may reduce bounce rate too (as I imagine a big proportion of people hit your site for their currency, then bounce off?) Andrew