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

docs style (bootstrap)

/* Base styles (regardless of theme) */
.bs-callout {
    margin: 20px 0;
    padding: 15px 30px 15px 15px;
    border-left: 5px solid #eee;
}

.bs-callout h4 {
    margin-top: 0;
}

.bs-callout p:last-child {
    margin-bottom: 0;
}

.bs-callout code,
.bs-callout .highlight {
    background-color: #fff;
}

/* Themes for different contexts */
.bs-callout-danger {
    background-color: #fcf2f2;
    border-color: #dFb5b4;
}

.bs-callout-warning {
    background-color: #fefbed;
    border-color: #f1e7bc;
}

.bs-callout-info {
    background-color: #f0f7fd;
    border-color: #d0e3f0;
}

.bs-callout-danger h4,
.bs-callout-danger a.alert-link {
    color: #B94A48;
}

.bs-callout-warning h4,
.bs-callout-warning a.alert-link {
    color: #C09853;
}

.bs-callout-info h4,
.bs-callout-info a.alert-link {
    color: #3A87AD;
}

.bs-callout a.alert-link {
    font-weight: bold;
}

That's new:

.bs-callout {
padding: 20px;
margin: 20px 0;
border: 1px solid #eee;
border-left-width: 5px;
border-radius: 3px;
}