Muther Hubbard said:
Does weapon finesse work with a quarterstaff in 3.5?
No (though some rules lawyers will argue that it should work for the off-hand attack with TWF).
Concerning 2 weapon-fighting: you get an extra attacks with the off-hand. Can the attack be used for a trip or a disarm (if you have that feat)? And when do you get extra attacks with your on-hand?
Yes, you can use special attacks with off-hand attacks. You get extra "on-hand" -- primary attacks -- as your base attack bonus goes up. So the 6th level ranger with Improved Two weapon fighting gets four attacks -- two primary (due to BAB +6/+1) and two off-hand (due to TWF/ITWF) at net attack bonuses of +4/+4/-1/-1.
As a lightly armored, two-weapon fighter, you have two things to consider:
1. Your poor AC
2. Your ability to deal damage
#1 is a direct consequence of your light armor (and if TWF comes from Ranger levels, you're stuck with it). Offset with high dexterity; consider Two-Weapon Defense and Expertise feats to directly increase AC. You might also consider the Spring Attack style of attack (taking advantage of Dodge and Mobility to increase your AC), but this has the distinct disadvantage of not allowing you the full attacks you need to deal damage with the TWF style.
To deal lots of damage in #2, you need to be able to produce a high volume of accurate attacks -- which means lots of opportunities for full attacks. You want to maximize the effectiveness of your weapon(s), offset penalties, and increase attack numbers. TWF, ITWF, and GTWF become a must (which requires high Dex as a prerequisite, unless you're a Ranger). The two weapons should be the same, and light weapons (short swords, for example) to take advantage of focus and/or specialization without taking too high a penalty -- and if your Dex is high, finessable weapons are a big plus.
I'd stay away from the special attacks (Trip, Disarm, etc) -- your specialty is in another area, and you're going to need all the feats you can.
Here's two character ideas (I'll present just 11 levels); I'm assuming human, with appropriate stats to meet requirements:
1. Ranger2/Ftr9: requires a high Dexterity (19 at 11th level); he's going to use two finesseable weapons (short swords)
Rgr1 -- Weapon Finesse, Weapon Focus: Shortsword
Rgr2 -- Combat Style: TWF
Rgr2/Ftr1 -- Two-Weapon Defense, Dodge
Rgr2/Ftr2 -- Combat Expertise
Rgr2/Ftr3
Rgr2/Ftr4 -- Weapon Specialization: Shortsword, Improved TWF
Rgr2/Ftr5
Rgr2/Ftr6 -- Improved Critical: Shortsword
Rgr2/Ftr7 -- Mobility
Rgr2/Ftr8 -- (Your choice -- could be Spring Attack, a special attack, or something else)
Rgr2/Ftr9 -- Greater TWF
2. Rgr11: requires a lower Dex (using Ranger levels to offset the Dex requirements) and focuses more on Strength using the quarterstaff (which has a distinct benefit of being able to be used as both two weapons and an two-handed weapon). You trade off being able to specialize for a while (take Ftr levels after Ranger 11), but gain damage from putting ability points in Str rather than Dex.
Rgr1 -- Power Attack, Weapon Focus: Quarterstaff
Rgr2 -- Combat Style: TWF
Rgr3 -- Two Weapon Defense
Rgr4
Rgr5
Rgr6 -- Combat Style: ITWF; Combat Expertise
Rgr7
Rgr8
Rgr9 -- Dodge
Rgr10
Rgr11 -- Combat Style: GTWF
Comparison at level 11 (ignoring stats, since we don't know what they are):
Rgr2/Ftr9: Full Attack of +10/+10/+5/+5/+0/+0 (plus Dex)
Damage: 1d6+2+Str each primary attack, 1d6+2+.5 Str offhand
AC: +1 TWD, +1 Dodge, +x Expertise (+4 vs. movement AoO)
Single attack is +12+Dex, damage 1d6+2+Str
Rgr11: Full attack of +10/+10/+5/+5/+0/+0 (plus Str)
Damage: 1d6+Str primary, 1d6+.5 Str offhand (+Power attack, if any)
AC: +1 TWD, +1 Dodge, +x Expertise
Single Attack is +12+Str, damage 1d6+1.5*Str (+ 2 x PA, if any)
Very similar. The first character will have a higher AC due to generally higher Dex, and deal damage well on full attacks, but loses out some when only making a single attack. The second chararcter will deal more damage if you move around a lot, due to higher average Strength (which offsets lack of Weapon Specialization), two-handed attacks, and ability to Power Attack for extra damage, but will have a bit lower AC due to less of a focus on Dex.
Mix and match between the two to get the features you want!