There are a lot of lowgrade attachments for lenses, sadly they do work but so poorly that they often end up degrading performance to a point where people greatly dislike what they get.
There are some gems though and I'd strongly suggest looking at the Raynox series macro filters/attachments. They are high quality and certainly good bits of glass worth considering purchasing. The DCR 150 and stonger DCR 250 are the most popular starting points, offering good magnification and a decent price.