Appraise

It's all about having sharp senses to notice the little things, and enough brains to appreciate their significance.

The art of estimating the value and effectiveness of things you find, people you meet, and creatures you fight.

Using can tell you: (1) Either on 'Market' or 'Open Market' value. Open Market is closer to what you'd fetch on tradenet, though as an upper limit. Remember to take supply-demand into account, and consider compromise in price if nobody is biting! No prices are set in stone.
 * How you stack up against a creature, combat-wise.
 * What equipment a creature has.
 * The weight of an object.
 * The approximate resale value of an object. (1)
 * The difficulty of installing a mod on a weapon.
 * Pretty much everything you'd want to know about anything, ever.
 * What kind of damage a weapon you are holding does and if it's damage scales with skills or stats better.

At lower levels of appraise, information obtained is often determined by a 'roll' of appraise. For example, one time you might only acquire a resale value of a weapon, and the very next roll you'd get all the information on how the damage spreads over the various types, etc. For this reason, tagging appraise is not recommended - any information you couldn't get off of the wiki/needed 'right away', you could just reroll for unless your appraise was at, say, 4.