I really like the Stouffer's spinach souffle but I don't think I've had it in years. My Yankee upbringing will only permit me to buy it when it's at least half-off price, and then I just can't justify turning the oven on for 45 minutes for a side dish. Isn't it great what you learn from your parents.
The Birds Eye creamed spinach is okay and it nukes just fine. I like a pinch of white pepper and a touch of grated parmesan to build out the flavor. The other frozen treat we like is their green beans with spaetzle and bacon. Nothing like real spaetzle and bacon, but for 3 minutes in the wave it's something different.
For a mac & cheese side dish we're happy enough with Annie's (the white cheddar variety), on sale, of course. Seems a little fresher tasting than most. Of course I prepare it with milk and butter, and sometimes a shake of parm.
Only other packaged side dishes we use are the Near East pilafs. They don't seem to be quite as salty as other rice mixes. There's an almond one we sometimes like with a piece of baked fish, and the lentil pilaf is pretty hearty with a piece of meat.