Где то див не закрыл.
Возможно так
<div class="slider1">
<iframe src="img/slider/slider.html" scrolling="no" style="border:0; width:99%; height: 249px; margin-bottom: 5px;"></iframe>
</div>
<div id="block_b">
<div class="title_block_b">
Тарифы
</div>
<div class="main_block_b">
<div class="service_block">
<img alt="Counter-Strike: 1.6" src="img/cs.jpg">
<div class="service_info">Counter-Strike: 1.6</div>
<a href="/servers/buy/game/cs" target="_top">
<div class="service_buy">
Подробнее
</div>
</a>
<div class="service_price">
7 руб.
</div>


<div class="service_block">
<img alt="GTA: MTA" src="img/mta.jpg">
<div class="service_info">GTA: MTA</div>
<a href="/servers/buy/game/mta" target="_top">
<div class="service_buy">
Подробнее
</div>
</a>
<div class="service_price">
1 руб.
</div>
<div class="service_block">
<img alt="GTA: SAMP" src="img/samp.jpg">
<div class="service_info">GTA: SAMP</div>
<a href="/servers/buy/game/samp" target="_top">
<div class="service_buy">
Подробнее
</div>
</a>
<div class="service_price">
1 руб.
</div>
</div>
</div>
</div>
</div>
<div id="block_b">
<div class="title_block_b">