I think this is what you are looking for:
Code:
<SCRIPT>
function checkIt(value){
if (value == "Other"){
document.getElementById("field").style.display = "";
}
}
</SCRIPT>
<SELECT name="country" id="country" onchange="checkIt(this.value);">
<OPTION value="">Option 1</OPTION>
<OPTION value="">Option 2</OPTION>
<OPTION value="Other">Other</OPTION>
</SELECT>
<BR>
<INPUT id="field" type="text" name="chosencountry" style="display:none;">
greetz