ピュアCSSだけで作るコーナーエフェクト:Pure CSS folded-corner effectのご紹介です。
via.http://nicolasgallagher.com/pure-css-folded-corner-effect/

対応ブラウザ:Firefox 3.5+, Chrome 4+, Safari 4+, Opera 10+, IE 8+
DEMO

CSS

.note {
   position:relative;
   width:30%;
   padding:1em 1.5em;
   margin:2em auto;
   color:#fff;
   background:#97C02F;
   overflow:hidden;
}

.note:before {
   content:"";
   position:absolute;
   top:0;
   right:0;
   border-width:0 16px 16px 0;
   border-style:solid;
   border-color:#fff #fff #658E15 #658E15;
   background:#658E15;
   -webkit-box-shadow:0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
   -moz-box-shadow:0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
   box-shadow:0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
   display:block; width:0; /* Firefox 3.0 damage limitation */
}

.note.rounded {
   -webkit-border-radius:5px;
   -moz-border-radius:5px;
   border-radius:5px;
}

.note.rounded:before {
   border-width:8px;
   border-color:#fff #fff transparent transparent;
   -webkit-border-bottom-left-radius:5px;
   -moz-border-radius:0 0 0 5px;
   border-radius:0 0 0 5px;
}