This is the Twitter Bootstrap documentation's callout css. Looks nice.

callout.scss

// Brands
$brand-primary: #3097D1;
$brand-info: #8eb4cb;
$brand-success: #2ab27b;
$brand-warning: #cbb956;
$brand-danger: #bf5329;

.bs-callout {
  padding: 20px;
  margin: 20px 0;
  border: 1px solid #eee;
  border-left-width: 5px;
  border-radius: 3px;
}
.bs-callout h4 {
  margin-top: 0;
  margin-bottom: 5px;
}
.bs-callout p:last-child {
  margin-bottom: 0;
}
.bs-callout code {
  border-radius: 3px;
}

/* Tighten up space between multiple callouts */
.bs-callout + .bs-callout {
  margin-top: -5px;
}

/* Variations */
.bs-callout-danger {
  border-left-color: $brand-danger;
}
.bs-callout-danger h4 {
  color: $brand-danger;
}
.bs-callout-warning {
  border-left-color: $brand-warning;
}
.bs-callout-warning h4 {
  color: $brand-warning;
}
.bs-callout-info {
  border-left-color: $brand-info;
}
.bs-callout-info h4 {
  color: $brand-info;
}