Imo, Fallout 3 is completely broken out of the box in terms of game balance. There are definitely some mods I would never play without. I ripped this list out of a thread I made about a year ago when I played FO3. You should be able to locate all of these mods at Fallout3nexus.com.
RH_Iron Sights - A mod that adds true 'down the sight' aiming. Makes combat a lot more realistic and enjoyable for me. I couldn't imagine ever playing FO3 without it now. Many thanks to skavenhorde for pointing it out to me.
DarNified UI - Makes fonts a lot smaller, and allows you to customize certain aspects of the HUD and menus.
Arwen's Reduced XP - Necessary to slow the hyper-fast levelling process.
Delay DLC - Delays activation of the DLCs by making it so you have to actually stumble upon those quests. Otherwise you get a ton of "new quest" messages as soon as you exit the vault, which I find unrealistic and immersion breaking.
Bobble NoBoost - Removes the overpowered stat bonuses from finding Bobbleheads.
No Enchanted Armor - Removes skill bonuses from armors/clothing. Your character will already be overpowered in this game without the additional skill bonuses.
Achievement Remover - Removes all achievements from the game. I found them to be annoying and immersion breaking. Some people like achievements though..
Karma Revamp - Overhauls the karma bonuses and reductions, so your karma doesn't fluctuate so drastically. It was way too easy to become good/evil in the vanilla game imo.
I also did some further editing with the UI mod. My compass doesn't show quest markers, landmarks, or NPCs. Also, I don't get a message telling me if I'm "hidden" while sneaking.
Finally, I changed the timescale to make it more realistic. In the vanilla game, an entire day/night cycle lasted only 48 minutes, which I thought was completely ridiculous. I'm using a setting that makes the day/night cycle last 3 hours.
Explanation. I think I used a setting of "8".