DP and I together collect shot glasses - also because they are inexpensive and easy to transport, and sometimes are really lovely. We also collect old books - specifically old bibles. DP was the pastor to the former owners of most of ours - pastor at a gay church in the 80s and 90s, she lost a lot of parishioners who had no family that would speak to them, and she keeps their bibles in remembrance. Some, though, are truly ancient (1800s) that she was given, again mostly by parishioners.
DP collects clipper ships - on mugs, models, paintings, you name it. We have a TON we need to get out and display after the move.
I collect blue Christmas ornaments. I've been buying one a year for myself since I graduated high school - my first was bought my senior year, come to think of it. I need a new ornament tree for them - I've long since filled the one I had.