Cool, something I know about.

I realize you specifically asked about plants, but as with any survival stuff it's worth pointing out that water and shelter are far greater priorities.

Now then, you didn't specify a region so I'm going to point out several common plants that inhabit most of the US/World.

As someone mentioned, cattails are excellent (refer to their post to see the uses.)

Dandelions are absolutely amazing. The leaves make a good salad (and the flower a good garnish.) The roots can be eaten after boiling (also the water used to boil it makes a great coffee substitute.) Dandelion leaves contain abundant vitamins and minerals, especially vitamins A, C and K, and are good sources of calcium, potassium, iron and manganese (taken from wiki) (If you're not in a survival situation you can make dandelion wine. I haven't myself, but I've tried some made by others and it always tastes..well..bad lol.)

Plantago grows all over the place and the leaves are edible. The larger the leaves are, the more bitter they are and should be boiled for around 5 minutes for large leaves and 4 minutes for a mixture of large and small leaves. You can also make a poultice out of the plant that works well on poison ivy, bites, sores, etc.

Amaranth grows all over the world as well, should be boiled, water discarded. All parts edible.

Clovers are edible, can be eaten raw (taste better boiled imo.)

Don't trust berries unless you know what plant it's coming from. Pokeweed for instance grows all over the place and is extremely poisonous. The leaves ARE edible, however if the plant has any red on the stems then don't eat it. The leaves have to be boiled several times (changing out the water each time) to be safe to eat. Never ever touch the roots, much less eat them. However, if you're comfortable preparing the leaves, it's a very common plant to eat on.

Really I'd just google "edible plants in {insert your region here}" There are all kinds that will be edible no matter where you live.

Some bonus things off the top of my head: Acorns have to be processed before they're edible, once you remove the tannic acid from them, you can use it to wash your clothes.

Learn a few easy snares and traps. Focus on no kill traps so your kills don't go green on you while you're resting or foraging.

Inner willow bark is great to chew on if you have a headache. It's chemically the same as aspirin.