We don't write up a whole lot of iPod Touch 4G cases, but Trident's Aegis case showed up in our offices and we thought it was worth calling out.
It's a double-layer case, with a soft silicone interior and hard polycarbonate shell on the outside that's got a nice rubberized finish. The silicone interior is thicker on the corners, which provides a little extra protection should you drop your Touch.
The Aegis comes in six colors and ships with a "scratch and smudge-resistant screen protector" as well as an audio extender in case your headphone jack doesn't fit into the headphone port (with the case on, the port is somewhat recessed, so some headphone jacks won't be able to slide all the way into the port).
All in all, this is a solid case that's got some toughness to it and also looks good. Its list price is $24.95 but we've seen it for closer to $20 in some online stores.