Logarithmic drops is an option in Hercules and rAthena you can enable for your drop rate formula.
// Logarithmic drops scale drop rates in a non-linear fashion using the equation
// Droprate(x,y) = x * (5 - log(x)) ^ (ln(y) / ln(5))
// Where x is the original drop rate and y is the drop_rate modifier (the previously mentioned item_rate* variables)
// Use the following table for an idea of how the rate will affect drop rates when logarithmic drops are used:
// X: Original Drop Rate
// Y: Rate drop modifier (eg: item_rate_equip)
// Y\X | 0.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00 5.00 10.00 20.00
// -----+---------------------------------------------------------------
// 50 | 0.01 0.01 0.03 0.06 0.11 0.30 0.62 1.30 3.49 7.42 15.92
// 100 | 0.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00 5.00 10.00 20.00
// 200 | 0.02 0.04 0.09 0.18 0.35 0.84 1.61 3.07 7.16 13.48 25.13
// 500 | 0.05 0.09 0.22 0.40 0.74 1.65 3.00 5.40 11.51 20.00 33.98
// 1000 | 0.10 0.18 0.40 0.73 1.30 2.76 4.82 8.28 16.47 26.96 42.69
// 2000 | 0.20 0.36 0.76 1.32 2.28 4.62 7.73 12.70 23.58 36.33 53.64
// 5000 | 0.50 0.86 1.73 2.91 4.81 9.11 14.45 22.34 37.90 53.91 72.53
//10000 | 1.00 1.67 3.25 5.28 8.44 15.24 23.19 34.26 54.57 72.67 91.13
//20000 | 2.00 3.26 6.09 9.59 14.83 25.49 37.21 52.55 77.70 97.95 100%
//50000 | 5.00 7.87 13.98 21.12 31.23 50.31 69.56 92.48 100% 100% 100%
Personally, I use an alternative logarithmic formula, which is just "extra chances at the item" so 5x drops on a 20% item would be 5 chances to get the item based on calculating probability. It works similar in effect, and it's not an arbitrary formula.
// Droprate(x,y) = 1 - ((1 - x) ^ y)
// X: Original Drop Rate
// Y: Rate drop modifier (eg: item_rate_equip)
// Y\X | 0.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00 5.00 10.00 20.00
// -----+---------------------------------------------------------------
// 50 | 0.01 0.01 0.03 0.05 0.10 0.20 0.50 1.01 2.53 5.13 10.56
// 100 | 0.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00 5.00 10.00 20.00
// 200 | 0.02 0.04 0.10 0.20 0.40 1.00 1.99 3.96 9.75 19.00 36.00
// 500 | 0.05 0.10 0.25 0.50 1.00 2.48 4.90 9.61 22.62 40.95 67.23
// 1000 | 0.10 0.20 0.50 1.00 1.98 4.89 9.56 18.29 40.13 65.13 89.26
// 2000 | 0.20 0.40 1.00 1.98 3.92 9.54 18.21 33.24 64.15 87.84 98.85
// 5000 | 0.50 1.00 2.47 4.88 9.53 22.17 39.50 63.58 92.31 99.48 100%
//10000 | 1.00 1.98 4.88 9.52 18.14 39.42 63.40 86.74 99.41 100% 100%
//20000 | 2.00 3.92 9.52 18.14 32.99 63.30 86.60 98.24 100% 100% 100%
//50000 | 5.00 9.52 22.12 39.36 63.25 91.84 99.34 100% 100% 100% 100%
And to relate to the topic, my favorite rates are anywhere from 3x - 10x exp and 1x - 5x drops. That way I can feel a sense of pride and accomplishment in my character(s).