First off, to modify any constants in these files you're going to want a hex editor; something simple like this one is ideal. Second, notation: 0x is a prefix meaning "hexadecimal"; for example, 0xFF is 255 in decimal. i is a suffix meaning "4-byte integer"; for example, 2000i would be encoded as 0xD0070000. f is a suffix meaning "4-byte single-precision floating point"; for example, 2000f would be encoded as 0x0000FA44. d is a suffix meaning "8-byte double-precision floating point"; for example, 2000d would be encoded as 0x0000000000409F40. The hex editor will take care of encoding values into hexadecimal for you, so just make sure you're encoding it as the correct data type. If you put an integer where a float should go, bad things could happen.

The format below is: <value> in <file>, <address> = <functionality>

-------- Visibility --------
3f in Freelancer.exe, 0x210530 = near plane of view frustum
250000f in Freelancer.exe, 0x210534 = far plane of view frustum (nothing will be drawn beyond this)
20000f in Freelancer.exe, 0x1C8910 = max distance at which ships can be seen (even if LODranges specify a higher distance)
100000000f in Freelancer.exe, 0x1D15B8 = square of max distance that a ship can be selected
6250000f in server.dll, 0x84ADC = square of NPC disappear distance in MP
25000000f in server.dll, 0x84AE0 = square of player disappear distance in MP
100000000f in server.dll, 0x84AE4 = square of grouped player disappear distance in MP
Another note on visibility, planets need LODranges specified if they are over about 1.5 million meters away (just add LODranges = 0, 1000000000 to the solararch entry and you'll be set).

-------- NPC spawning/behavior --------
1775d in content.dll, 0x11AC80 = initial NPC max spawn distance in SP and MP (such as after respawn)
2500d in content.dll, 0x11AC68 = NPC max spawn distance in SP and MP (following initial spawn)
2500f in content.dll, 0xD3C3E = maximum distance that NPCs will persist in MP (I don't recall if it applies to SP)
3750f in content.dll, 0x591B6 = distance over which NPC spawning will ignore density cap in SP; as a general rule, make it 1.5 * NPC max spawn distance
7500f in content.dll, 0x116A68 = distance over which NPC spawning will ignore density cap in MP; as a general rule, make it 3 * NPC max spawn distance
5000f in common.dll, 0x1407E0 = max range at which NPCs will engage enemies; don't forget to increase the range at which NPCs will engage in pilots_population as well (attack_preference under JobBlock)
5000f in content.dll, 0xD0510 AND 0x11AD10 = max range at which NPCs will go hostile upon seeing their allies go hostile (AND means both values must be changed)
500i in content.dll, 0x6C790 = max range at which NPCs will scan your cargo
2500f in content.dll, 0x6CA37 = max range at which NPCs will initiate scan of your cargo

-------- HUD --------
2000i in Freelancer.exe, 0xD2C02 = distance over which kilometers rather than meters are displayed (in the contact list)
2000i in Freelancer.exe, 0xEEF90 = distance over which kilometers rather than meters are displayed (target bracket in space)
10000i in Freelancer.exe, 0xD2C32 = distance over which fractions of kilometers are not displayed
99999i in Freelancer.exe, 0xD2C94 = distance over which "far" is displayed rather than an actual distance
999i in Freelancer.exe, 0xD597A = speed over which "---" is displayed
0x0F85->0x90E9 in Freelancer.exe, 0xD5936 = remove cruise speed display limit of 300 (in other words, change 0x0F85 to 0x90E9; this hack was found by someone else)
0.6f in Freelancer.exe, 0xDFC09 = speed at which target crosshair closes on subtarget in wireframe view (make smaller for faster)
0.8d in Freelancer.exe, 0x1D7E50 = width of bars for energy, shield and hull gauges (for example, set to 1.0 to make the bars solid)
1.25f in Freelancer.exe, 0x1C9804 = multiplier for distance at which brackets around non-targeted ships in space will appear (1.25 = ~3km, 37.5 = ~93km, etc)
3.0f in Freelancer.exe, 0x1D7964 = seconds between updates of the distances in the contact list

-------- Misc --------
1000f in common.dll, 0x18ADB4 = max dock acknowledge distance
10000f in common.dll, 0x13F45C = max docking initiation distance
0.8f in Freelancer.exe, 0x1D6D38 = resale % for ships
9999999f in Freelancer.exe, 0x1CAEE8 = maximum value of any single good
0.9f in common.dll, 0x18C5D4 = Reputation over which will give you affiliation with that faction (faction name appears in space before your name)

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Visual effects: These are kind of a bizarre hack I came up with for making vis_beam effects appear beyond 2k, since I couldn't track down the constant(s) responsible for them disappearing. If you don't need to make vis_beams appear at long range then don't bother with this, since it's much easier to work with effect_types.ini.
For any of this to work, you must first do this:
0x45->(something else) in Freelancer.exe, 0x1C8890 - alter the 'E' of 'EffectType' to make the parser not recognize EffectType from effect_types.ini and force default render distances.

Once this is done you can directly edit the defaults. They more or less correspond with the values you would normally set in effect_types.ini. There are default entries for every effect type, but I found that I only needed to change three. However, they're all bunched together in Freelancer.exe, so it shouldn't horribly difficult to identify the rest of them.
EFT_EXPLOSION_LARGE (limited duration):
100f in Freelancer.exe, 0x212AF0 - on-screen radius of effects, affects size-on-screen-based cutoff (make larger to increase cutoff range)
15f in Freelancer.exe, 0x212AF8 - max run time of effects
100f in Freelancer.exe, 0x212B00 - unknown, likely the first value of pbubble
500f in Freelancer.exe, 0x212B04 - visual cutoff range of effects (second value of pbubble)

EFT_WEAPON_LARGE_PROJ (infinite duration):
50f in Freelancer.exe, 0x212C58 - on-screen radius of effects
-1f in Freelancer.exe, 0x212C60 - max run time of effects
100f in Freelancer.exe, 0x212C68 - unknown, likely the first value of pbubble
500f in Freelancer.exe, 0x212C6C - visual cutoff range of effects (second value of pbubble)
15f in Freelancer.exe, 0x213158 - on-screen radius of vis_beam effects
100f in Freelancer.exe, 0x213168 - unknown, likely the first value of pbubble for vis_beam
500f in Freelancer.exe, 0x21316C - default vis_beam cutoff range (second value of pbubble for vis_beam)

EFT_DAMAGE_LARGE_SMOKE (infinite duration):
20f in Freelancer.exe, 0x212DE8 - on-screen radius of effects
-1f in Freelancer.exe, 0x212DF0 - max run time of effects
100f in Freelancer.exe, 0x212DF8 - unknown, likely the first value of pbubble
500f in Freelancer.exe, 0x212DFC - visual cutoff range of effects (second value of pbubble)

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Those are all the hacks that I have found (except for the cruise speed one, that was somebody else). All have been extensively tested, so they definitely will work. The real trick will be making good use of them. May the zomgbay haxx be with you.