A good way of stealing good weapons, chemicals, clothes and crappy 食物 in Fallout 3 without needing to hide yourself and rely on your Sneak and Lockpick skills, is 由 going to Rivet City's market.
Of course 你 will lose Karma if 你 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 你 "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 你 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 你 grabbed on the floor.Well yeah, the angry looking character will pull a face at 你 on your screen and 你 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 你 can to the water beggars 你 encounter around the Wasteland, in order to raise your Karma. It's the easiest thing on Earth to find these beggars. If 你 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 随意 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 你 go. You've just left 拱顶, 保险库, 跳马 101 and already have a combat shotgun, chinese assault rifle, sledgehammer and many 更多 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 你 probably realized 由 now, 你 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?