iframe, widget, external search

[woopwoop]

I'm planning on creating a simple 1 field and submit button form that will have method='get' and be shown on other people's websites. The search function will search my site and I'm wondering the best way to do this.

I guess I could:
1. Create an iframe and have the search box hosted on my site (not sure how to do this)
2. Give them the html code, but then the box might change or look how I want it to.
3. Some other widget


Has anyone done anything like this and know the best way?
Am also thinking about SEO, and the form will probably have a link on it back to my site's homepage, as well as the action going to a page on my site. What impact does an iframe or the other methods have on seo?

Thanks for any help!