Edit in JSFiddle

<div class="main_block">
    <div>
        <div class="block">Выравнивание - блок по вертикали</div>
    </div>
</div>

.main_block {
    display: table;
    width: 100%;
    height: 200px;
    background: #f00;
}
.main_block > div {
    display: table-cell;
    vertical-align: middle;
    background: #eee;
    height: 30px;
}
.block {
    background: #ccc;
}