Acoustibuds are little rubber things that one wraps around standard earbuds to make them more comfortable. A series of concentric fins means they snuggle nicely in your ears: great for joggers or anyone else fond of the form, but not the feeling, of buds.
They cost $15 a pack, and for that, you get two pairs in two sizes.
The downside is somewhat obvious: by putting 1/2" rubber tubes between your ears and the buds, the audio is just a smidge off. This is marketed as an improvement in quality by Acoustibuds, and it does beef up the perceived quality of cheap, tinny buds. Remember, kids, you can't magically recreate data that's already gone, you can just polish it up so it doesn't sound so bad.
On decent ones it makes it a little more muffled, a little distant, but really, if you're using earbuds, you're not going to be that bothered. Buy them for comfort, if you buy them at all.
For basic iPod pairs, I can't decide if I like it more: the feel is nice, but I think it might be less danceable.