You can use a normal anchor to submit a form, but that requires the use of javascript, which is easily disabled.
Another way is using the input/image type, but that's functionally very similar to a standard submit button. In theory, you could specify whatever text you wanted in the alt attribute, point the src attribute at an invalid URL, and it should just show the alt text (which should work like a link... in theory--keep in mind I've not actually tried this.)\
Edit: NM, just tested the following:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" >
<head>
<title>Link submit</title>
</head>
<body>
<form method="get" action="#">
<p>
<input type="image" alt="This link should submit" src="completelyinvalidurl.gif" />
<input type="hidden" name="testvar" value="true" />
</p>
</form>
</body>
</html>
...It works beautifully in Firefox, but in IE, it shows a disgusting image not found placeholder with the alt text inside.... not the desired effect. Oh well, I tried 🙂