The custom among all Jews (as far as I know) is to spill a drip of wine for each of the plagues as they are mentioned in a list in the haggadah. This custom is not mentioned anywhere in the Talmud (as far as I know). What is the earliest source to mention this custom? Does it encourage or discourage the practice? What reason does it give?
The דרכי משה ( in אורח חיים ס' תעד ס'ק יח ) brings the custom based on the מהרי'ל and the custom of the מהר'ש. He also states that the מהר'ש based it on the ספר אבי'ה - presumably the ראבי'ה. There are two things being symbolized. The use of the finger symbolizes the 'finger of G-d' and the number of times has a gematria of 16. The דרכי משה explains this to be referring to the faces of the חיות or a gematria linking it to G-d's sword. The משנה ברורה (in ס'ק עד on that סימן) has it referring to the name of G-d (י-ו). The specific finger to use is brought by the מגן אברהם (in ס'ק כח) as the קמיצה (ring finger) based on a מדרש that G-d used his ring finger to deliver the plagues. The שער הציון on that piece also says that it's better to spill from the cup if you will be disgusted drinking from a glass you put your finger in and that some authorities say that its preferable to spill whatever the case.