For the most part, I don't drink a lot of sweet table wines. Not because I don't like sweetness, but rather because the sugar is often added to cover up a mediocre product, and I think some almost border on Kool-Aid levels of sweetness.
Be that as it may, if you like Riesling you may want to try some German Rieslings. In Germany, the sweeter wines tend to be the most expensive. The reason for this is that the grapes are left on the vines longer, which exposes them to the risk of being destroyed by pests, bad weather, crop diseases, etc. If you're looking for a semi-sweet wine, then the style you want to seek out is Kabinett or Kabinettwein. Spätlese (late harvest) is the next step up on the scale.
If you want to stay with domestic wines, most under-$20 Moscato, Riesling, and Gewürztraminers will usually be somewhat sweet.
The wines that Craig mentions above are dessert wines, which is a completely different class of wine that includes Port, Sauternes, Ice Wine (or Eiswein), Vin Santo, domestic Late Harvest wines, etc. True to the name, they are often served in place of (or in addition to) dessert. You probably wouldn't want to drink one of these with, say, a steak. But they are delicious on their own.