What AoM said. Crappy woodland scenics also mixes nicely with both inks and paints to make a lovely (if translucent) scum.
As ever, I can hardly recommend "fine art" gels/mediums/effects, either. Look at michaels/A.C. Moore/art supply store for lots and lots of fun things to mix with paints or inks. (Inks are better, generally) I'm a big liquitex fan (natural sand and string gel, heck yeah!), but Golden makes some lovely stuff, too.
Check this guy out-- he's embellished with modeling paste, string gel, and gloss medium. Not too shabby, eh?