What substances are good for spherification with algin?

Basically anything that's liquid. Fruit is very common, but we've also done herbs (mint and sage specifically). I've also had vegetable spheres like pureed peas and beets as separate spheres. At restaurants, I've had beer sphere's served with pretzel (at Cyrus) and I've seen cointreau spheres used in a drink.

They pop like caviar, and that's what releases the flavor, so they either need to be quite large or have a strong flavor. Otherwise the taste is lost and you're left with just the (fun) "texture". It also often helps to use some food coloring if your liquid isn't a strong color. With small spheres, light colors will disappear and you're left with relatively transparent spheres. I'd also recomend trying reverse spherification (i.e. mix calcium chloride with your liquid and then drop in to an alginate bath), as this doesn't use your flavored liquid to make the shell, leaving you with more "pop".

So look for a strong flavor and a nice pairing. Make it easy to get the spheres in to your mouth. It's easier if the spheres are on another piece of food, as cutlery will often break the spheres. The best thing about spheres is presentation and the caviar pop, so use that experience and look to your advantage and feature it in whatever you're serving. Here's my Sushi Dessert revolving around spherification.

(Cold) intensely-flavoured soups also work very well, as long as they are rigorously strained to remove any particulate matter. When I was in culinary school I did a stage at a (now defunct) restaurant which served a carrot-ginger soup (cook carrots & ginger together over low heat until tender in just enough carrot juice to cover. Puree as fine as possible, strain overnight through a fine chinois) which was 'reverse' spherified, then topped with candied cilantro stems (made exactly as you would candy anything else, but only for about 40 seconds, chilled rapidly to prevent loss of colour), coconut powder (thick cream from can of coconut milk blended approx 1:1 with maltodextrin) and Murrey River salt. Delicious.

On YouTube one of Achatz' chefs presents him with a gin & tonic which included cucumber spheres. I had a spectacular intermezzo once; raspberry sorbet topped with basil spheres (no idea how they made them).

Oils can be a fantastic option as well: very good olive oil, pistachio walnut, hazelnut...

Alcohol :-)

El Bulli (and The Aviary using El Bulli's recipes with permission) serve a cocktail called the "disappearing barbapapa". It's a modern take on a pina colada that starts with spherized rum in coconut milk with cotton candy at the top of the glass - then at the table, pineapple juice mixed with cider is poured through the cotton candy and the drink turns into a pina colada where rum balls explode in your mouth.

I've heard of it being done with soy sauce.

The "Hydrocolloid Recipe Collection" has 17 recipes beginning on page 56, culled from various online sources:

http://blog.khymos.org/recipe-collection/