<input type="checkbox" id="ckb_1" /> 
<input type="checkbox" id="ckb_2" disabled="true" /> 
<input type="checkbox" id="ckb_3" /> 
<input type="checkbox" id="ckb_4" /> 
<input type="button" id="btn" value="点击">

$("input[type='checkbox'][disabled!=disabled]").attr("checked",true);

$('input:checkbox[disabled!=disabled]').attr("checked",true);

$(':checkbox:enabled').attr("checked",true);

$("input[type=checkbox]").each(function(){

if ($(this).attr("disabled") != "disabled") {

$(this).attr("checked",true);
}
});

<ul>
<li >第一行</li>
<li class="showli">第二行</li>
<li class="showli">第三行</li>
<li>第四行</li>
<li style="display:none">第五行</li>
<li class="showli">第六行</li>
<li>第七行</li>
</ul>

$("ul li[class=showli]:eq(0)").css("background":"red");

$("ul li:visible:eq(4)").css({"display":"blaock","background":"red"});

$("ul li.showli:eq(1)").nextAll("li:visible:eq(1)").css({"display":"block","background":"red"});