Anything I like enough, once finished, gets zipped up and goes on an external drive and is subsequently never touched again. Anything I haven't completed is kept until I do.
Usually I only keep games that I'm actively interested in, and are actively being developed. Others I remove or I only check out periodically when new updates have come out.
Games that I have spent many hours in and have replayability, I like to keep depending on size. Something you can fire up at any time.
Other games that have a strong story or leave a big impression, I like to keep if it is small but mostly just end up keeping it in your memories. I like the idea of keeping an empty folder or file to help remember when you played it (mostly for very memorable games).