While conversing, I sometimes find myself, after listening to what my partner has to say, responding with phrases such as "I see" and "Okay" to indicate that I haven't lost interest but I either don't have a point to add, or the other person hasn't finished talking. Is there a word, preferably informal, to describe this kind of response?
I think phatic is the word. A phatic expression is one whose only function is to perform a social task, as opposed to conveying information. Utterances such as hello, how are you? and nice morning, isn't it? are also typically phatic.
Depending on context you might say that you give "laconic replies"
laconic (Of a person, speech, or style of writing) using very few words. ODO
Origin - Mid 16th century (in the sense 'Laconian'): via Latin from Greek Lakōnikos, from Lakōn 'Laconia, Sparta', the Spartans being known for their terse speech.
A Little History - It is named after Laconia, the region of Greece including the city of Sparta, whose inhabitants had a reputation for verbal austerity and were famous for their blunt and often pithy remarks.
The following quote illustrates why "Laconic" was coined to refer to those people who give ah... laconic replies: After invading Greece and receiving the submission of other key city-states, Philip II of Macedon sent a message to Sparta: "If I invade Laconia you will be destroyed, never to rise again. If I invade, we will kill most of the men, make some of them slaves and rape your women." The Spartan ephors replied with a single word: "If" (αἴκα). Subsequently neither Philip II nor his son Alexander the Great attempted to capture the city. from Wikipedia
Ps. Words in italics: I couldn't find the reference though I've heard the story that way more than once.