Which Tumblr themes handle

 tags well

I've got another idea.

{block:Tags}<a class="tag" href="{TagURL}">{Tag}</a><span class="tagtail"></span>{/block:Tags}

This will add a span with the "tagtail" class after every tag.

Then, add this css:

span.tagtail + a.tag:before {
   content:", ";
}

This will select every tag anchor that comes after a tagtail span (so every one but the first). This way, we avoid the need to use the first-child or last-child selectors.

UPDATE:

If you want to ensure that the commas are styled apart from the anchor, I suppose you could also do:

{block:Tags}<span class="taghead">, </span><a class="tag" href="{TagURL}">{Tag}</a>{/block:Tags}

and then style it like:

span.taghead { display:none; }
a.tag + span.taghead { display:inline; }

However, this might need tweaking because browsers differ on overriding rules.

Include the following code:

{block:HasTags}
   {block:Tags}
       <a class="tag" href="{TagURL}">{Tag}</a>
   {/block:Tags}
{/block:HasTags}

in the block for each type of post. So you will have this seven times: in {block:Text}, in {block:Audio}, etc.

Put this right before the closing of the block (so right before {/block:Text}, for example) to have the tags show up at the end of the post.

Tumblr provides some documentation on this here.