A good way of stealing good weapons, chemicals, clothes and crappy food in Fallout 3 without needing to hide yourself and rely on your Sneak and Lockpick skills, is by going to Rivet City's market.
Of course you will lose Karma if you steal things, but i'll explain how to alleviate that over time. Now, to steal anything publicly displayed on the stalls: Grab the object (grab it, don't press A and take it into your inventory, obviously. It's when you "levitate the object"...you know how to do that, right?) and take it to any shadowy part of the market, where your status is "hidden". There is a good shadowy part left from where you enter (the main entrance, door leading back to the Capital Wasteland), which is covered on top, casting a good barricade of darkness around you. Once you're there, release the weapon you grabbed on the floor.Well yeah, the angry looking character will pull a face at you on your screen and you will lose karma.
The simplest and most practical way of raising your karma is asking your Mister Gutsy for purified water and giving as many water as you can to the water beggars you encounter around the Wasteland, in order to raise your Karma. It's the easiest thing on Earth to find these beggars. If you don't know where they are located:
There is Carlos, lying outside Rivet City;
Micky, just outside Megaton's airplane-made gate;
And also Ben Canning. This guy appears as a random event, but the last time i saw him was just SOUTH of the Republic of Dave, lying behind a house (probably a MDPL power station. Anyways, it is exactly SOUTH of the Republic of Dave). He can also be found North West of Megaton, lying in between some rocks (he is found at the center of an aglomeration of big boulders).
(You can also complete quests to earn good karma)
So there you go. You've just left Vault 101 and already have a combat shotgun, chinese assault rifle, sledgehammer and many more great and powerful weapons! This stealing strategy i just presented is ideal for people who aren't afraid of lowering their karma, but definitely works with people who are, too! As you probably realized by now, you can actually perform this kind of stealing anywhere else (although the best place is in the Rivet City Market. Have a nice time stealing...or should we call it surviving?