<div class='noop-searchbox' id='noop-searchbox'>
<form action='/Buscador' id='noop-searchform' method='get'>
<input id='s' name='q' onblur='if (this.value == "") {this.value = "Buscador...";}' onfocus='if (this.value == "Buscador...") {this.value = ""}' type='text' value='Buscador...'/>
<button id='sbutton' type='submit'>
<span id='simg'/>
</span></button>
</form>
</div>
<style type="text/css">
#sbutton {
background: -moz-linear-gradient(center top , #4D90FE 0%, #4787ED 100%) repeat scroll 0pt 0pt transparent;background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#4d90fe), color-stop(100%,#4787ed));
background: -webkit-linear-gradient(top, #4d90fe 0%,#4787ed 100%);
background: -o-linear-gradient(top, #4d90fe 0%,#4787ed 100%);
background: -ms-linear-gradient(top, #4d90fe 0%,#4787ed 100%);
background: linear-gradient(top, #4d90fe 0%,#4787ed 100%);background-color: #4d90fe;
border: 1px solid #3079ED;
border-radius: 2px 2px 2px 2px;
color: # EDA0A0;
height: 27px;
min-width: 76px;
padding: 0 21px;padding-bottom: 2px;
}
#sbutton:hover{ background-color: #357AE8;
background-image: -moz-linear-gradient(center top , #4D90FE, #357AE8);background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#4d90fe), color-stop(100%,#357ae8));
background: -webkit-linear-gradient(top, #4d90fe 0%,#357ae8 100%);
background: -o-linear-gradient(top, #4d90fe 0%,#357ae8 100%);
background: -ms-linear-gradient(top, #4d90fe 0%,#357ae8 100%);
background: linear-gradient(top, #4d90fe 0%,#357ae8 100%);
border: 1px solid #2F5BB7;
box-shadow: 0 1px 1px rgba(0, 0, 0, 0.1);}
#simg {
background: url("https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhYV8y_UtcyZpeuRZ6qO7_158A4aPAZqzSjWvxVZenC_HWmXqiufgE2wRjAvnn3EE35L1NJgSooNTD9VahD_7k1rPabSD6qY3qa5NM1WeXg79UmXENQ7s1k_2_kOXIC1YeadOqduVRZJms/s1600/searchicon.png") no-repeat scroll 0 0 transparent;
display: inline-block;
height: 14px;
margin: 0;
width: 17px;z-index:101;
}#s:hover {box-shadow: 0 1px 2px #C1C1C1 inset;}
#s {border: 1px solid #BCBCBC;border-radius: 4px 4px 4px 4px;font: italic 14px times New Roman;padding: 4px 2px;width: 146px; -moz-transition: width 0.2s ease-in 0s; -webkit-transition: width 0.2s ease-in; -o-transition: width 0.2s ease-in;}
#s:focus {width: 200px;}
.noop-searchbox {
display: block;
position: fixed;
left: 0;
top: 1%;
max-width: 250px;min-width: 200px;
z-index: 100;
}
</style>
podéis modificar los colores, si como la esquina que quieres que aparezca y la altura y ancho de cuadro