<p>I've created a list using the SharePoint (MOSS 2007) Issue Tracking List.  A "Comments" field is automatically created in this list.  The Comments column has extra functionality that provides a sort of history/log whenever an edit is made to a list item.  Unfortunately, a comment entry is created even when an insignificant edit is made.  For example, I could edit the title of the list item and leave the comment field blank.  This results in a blank entry being saved for the Comments field (except the date/time of the edit and the person making it).  If there are several edits made (with the Comments field left empty), then several blank entries appear to be stored in the data (for the Comment field).  Is there a way to not save a comment entry if it's blank?</p>

<p>Also, is there a way to have a Comments field in a Custom List?  This column seems to only be available in the Issues Tracking List.</p>

The "Comments" field is actually an "Append-Only Comments" Column .  You should be able to add this to any custom list.  (You will need versioning turned on on the list to operate in the same manner as it does in issue tracker.)
As far as not getting "blank" entries, the only way to do this, would be to turn off "Append Changes to Existing Text" option on the column under list settings.  Doing this would turn it into just another multi-line text box.
I think you are missing the point of this column.  After an edit is made, you can click on the Date/time in this column and it will show you that version of the list item.  I realize that you could do this in version history as well, but this gives you a quick link inside the item.  Also, trained users should be documenting what they changed inside this box.  Might help to rename the column to "Note Changes Made Here." :)
Good Luck