This mod from Koldorn makes it so that you need more XP to gain levels.
With this mod you need more XP to gain levels and also get less XP than a normal game for combat kills and other things. So you level up much much slower, giving the freedom to play slowly and not hit the level cap for a long time.
Timeslip for FOMM
TheMandrake for original levelling mod
Bethesda for Fallout
Unzip with 7zip.
Place 'Slower Levelling.esp' in your Fallout 3/Data folder.
Launch with launcher and tick new esp under Fallout3.esm
Based on 'Better Pacing mod' by TheMandrake (http://www.bethsoft.com/bgsforums/index.php?showtopic=901886) but without all the other stuff unrelated to XP gains. Also this mod increases the amount of XP required to gain levels (ibumpXP increased to 300) aswell as descreasing XP from combat and other activities.
This mod does NOT touch walking speed or radiation in any way, its ONLY a slower levelling mod unlike the 'Better Pace mod project' which has a bigger scope.
Difference between the mods Slower levelling and Slow levelling. Both use different iXPbumplevel values. Everything else is the same apart from this one attribute. Here is a quick rundown of the stat in question:
Slower levelling - iXPbumplevel 300
Slow levelling - iXPbumplevel 225
normal game - iXPbumplevel 150
So Slower Levelling.esp requires twice as much XP for level gains (amount of XP required to get from one level to the level above) as normal game and 'Slow Levelling.esp' is a middle ground. BOTH files have the other XP tweaks where you get much less XP for all your actions.
Pick which esp you want to use and use ONLY that one.