I'm starting to get some gold, and was wondering what skill I should look into purchasing next.

I like CLEAVE, its quite effective since you get bombarded my multiple mobs a lot. At 33, I still use it more than any other skill.

I agree. Personally, I map Butcher to my left mouse key and cleave to my right...

~Ripper

I have it mapped the same, Butcher/Cleave.

I have <200 mp so I run out pretty fast, but I usually just keep going, unless I see a glowing monster.

Other than that, I've only really just started using skills when I was sissirat running. All the monsters were did poison damage, so I picked up the ranger poison resistence (level3). Wow it made a huge difference in the amount I needed to heal. So any res would be good, if you know what your fighting.
Retrobution I hear good things about, but I didn't really see a difference.
2handed attack speedbuff is useful(1handed probably as well) but again I only find myself using it before a boss.
I bought levels of the stomp, because I had it and was rich at the time, it's useful to stun the enemys a second so I can heal.
I also grabed the ranger speed buff, just to run passed cleared floors faster. (to find a quest giver for example) But it was also useful, if I'm low on hp i can cast it and run cicles around my foes until I recharge. Outrun the mobs, and dodge the casters/rangers.

Really it's just upto how you want to play.

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Retrobution I hear good things about, but I didn't really see a difference.

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Retribution is nice until you hit the level bump, enter a zone that is +5 levels or more to you and spell damage (which includes Retribution) will become 100% useless. Reflecting 75% of the damage dealt to you in melee can help thin out crowds of fodder and will speed up some of the boss encounters.

The skills I've bought (and used) on my fighter are:
Butcher, Cleave, Stomp, Thickheadedness, Retribution, Unnerving Shout, Acidic Blight, the two attack speed buffs, Speed of the Hare and the various resistance buffs.