“Block Code” Plugin/PHP Tag?

Is there any way I could have the equivalent of an #include for a block of HTML that I want to include in a separate file? Say, a part of my right sidebar has some stuff I want to update frequently but I don’t want to go to the index.php template and change it by hand every time, so I have an HTML file separate just for that and use a PHP #include equivalent or something? I wouldn’t know where to begin with PHP doing that but anyone have any suggestions?
I used to use those with MT include tags but I sort of miss those.