I've only used this in firefox, and it assumes that prototype.js is loaded on the page you're using. Furthermore, it only fills in textarea, input text, and radios. Very niche! I just am putting it up here because otherwise I'll lose it and have to spend 10 minutes of my life writing this code again.
javascript:$$("input[type=text], textarea").each(function(f){ f.value = f.name + " random value"}); $$("input[type=radio], input[type=checkbox").each(function(f) { f.checked = true }); undefinedSimilarly, here's a bookmarklet to clear forms:
javascript:$$("input[type=text], textarea").each(function(f){ f.value = ""}); $$("input[type=radio], input[type=checkbox").each(function(f) { f.checked = false }); undefined 