Who can you think of?

There was Mr. Haney's wife, who he may or may not have divorced at some point during the series...

There was Mr. Kimball's mother, who (according to some dialogue) Oliver and Lisa had met a few times before. According to Oliver, she "has all her marbles" as opposed to Hank. Hank also reportedly lived with his mother, though we never got to see their home. In one of his earliest appearances Mr. Kimball says Sarah (the operator) is his mother, but this was probably a mistake.

Eb's real parents, who live in Appleville a few hundred miles from Hooterville.

Old Man Gronyitz (Lisa's father, the King).

Alf and Ralph's mother.

The "Words People," as Lisa might describe them.

Sarah is also Mr. Trendall's mother, right? So theoretically Kimball and Trendall are (half) brothers.

Didn't Eb have an imaginary friend - Herbie or something?

I'm going with Sarah being Trendall's mother as official canon. As far as I know the only mention of Sarah being Hank's mother was early in the first season before the character of Trendall was even introduced, and was never alluded to again.

Eb's friend was Charlie. Lisa's was Natasha and Oliver made up Homer so he wouldn't be left out. No one could see any of them and it was revealed to be inspired by a rerun of Our Miss Brooks.