Caspr is a configurable assembler program, originally developed for an undergraduate course at URI. While fairly simple, it makes quick work of small assembler jobs for student microprocessors, and is very easy to port by means of a short configuration file.
It has seen some use outside of URI (link). And while not as featureful as some other assemblers, it makes quick work of small jobs, such as those in HDL processor design.
For those interested, the original lab handout is included in the doc directory.