I sometimes find myself using gravy sauce powder mixes to increase the volume of "au jus" (I know, it's a contradiction) from meat roasts, pan fries, etc. As for soup mixes, I tend to use them creatively by taking "cream of [blank]" mixes, add less than the recommended volume of liquid, and use them as thick sauces, for example, a mix of cauliflower/broccoli/carrots/brussel sprouts. In essence, a lot of doctoring and frankensteining ;-)
I use Knorr mixes almost all the time and really haven't bothered researching other brands as I'm very happy with Knorr. But I'm wondering if you CH-ers have any good ones to share, in case I'm missing out on something.