Any vegetable you like steamed or boiled works well in the microwave. It doesn't give you the Maillard reaction, caramelization and toasty flavors you get from roasting, though for long-cooking items you can get some of that by adding fat.
Like JoelF, I use the microwave for steamed redskin or white potatoes, and I also use leek's method of par-cooking baking potatoes before finishing them in the toaster oven. If you start with small white potatoes or potato chunks, micro-steam them to almost tender, then roll them around in oil and a lot of paprika and nuke them some more you can create a decent approximation of oven-roasted spuds (shortcut this process even more by starting with canned potatoes).
It's also a good way to make
German potato salad.
I prefer
roasted corn over steamed or boiled, so I'm not fond of the microwave for corn on the cob (also, I find that given the inside-out way microwaves work, the cobs get way too hot before the corn itself cooks, making the finished corn hard to handle).
The microwave is also great for:
I have also pot-roasted brisket in the microwave (adapted from the recipe in Barbara Kafka's
Microwave Gourmet, a book I highly recommend to anyone who wants to use a microwave as more than a reheating device). It works well, but like most brisket recipes, tastes better the next day. So it's not a time saver, though it does avoid heating up the kitchen, in case you crave braised brisket in the summer.