CSS: how do I create a gap between rows in a table?

table {
    border-collapse: collapse;
}

td {
    padding-top: .5em;
    padding-bottom: .5em;
}

The cells won't react to anything unless you set the border-collapse first. You can also add borders to TR elements once that's set (among other things.)

If this is for layout, I'd move to using DIVs and more up-to-date layout techniques, but if this is tabular data, knock yourself out. I still make heavy use of tables in my web applications for data.

All you need:

table {
    border-collapse: separate;
    border-spacing: 0 1em;
}

That assumes you want a 1em vertical gap, and no horizontal gap. If you're doing this, you should probably also look at controlling your line-height.

Sort of weird that some of the answers people gave involve border-collapse: collapse, whose effect is the exact opposite of what the question asked for.