Many of my faves have already been mentioned. Here are a few others you might want to watch for:
Quady 'Starboard' - http://tinyurl.com/d4ufcy
IMO a heckuva nice US dessert wine, named Starboard when the European market objected to Quady calling it Port. Their 88 is usually what I buy.
Hogue Late Harvest Riesling
Late Harvest is roughly the same as a Spatlese (depending on who's making it) Hogue LH is an excellent value - I buy it at Costco and it's a good 2nd bottle to open w/a cheese plate. I've also poached pears in this and had good results, or splashed the last part of a bottle over a fruit salad. FWIW I liked it better several years ago before ownership changed, but then again...I liked ME a lot better back then as well.
OZ tawny ports have been mentioned, another contender is the Penfolds Club tawny, and their Club Reserve tawny...both are good bargains, the reserve is well worth the extra $. Their Grandfather tawny is great, but frankly priced out of my league. Get the very tasty Club Reserve and spend the money you save on bourbon.
Niepoort LBV and tawny when I find a sale.
Let me also mention...Missouri dessert wines. They can't grow very many of the classic wine grapes in MO but they can grow some excellent hybrids. Those hybrid grapes may never push the classics off the shelf when making dry wine, but they CAN make some top notch dessert wines. Doug Frost, a KC wine writer I respect, has said that Missouri dessert wines are some of the best he's ever tasted anywhere. Augusta's Vintage Port is a personal fave, Stone Hill Late Harvest Vignoles another. So give them an honest try if you get the chance, you'll probably be surprised. They seldom beat the Aussies or Hogue Late Harvest for value, but if you're passing thru the area keep them in mind.