Every year or so I have my fun with Quake 3 and UT99 with bots. They might not be as smart as a human opponent but playing on Nightmare and Godlike difficulty brings back that twitchy adrenaline infused moments I enjoy so much from those games :)
A potential side project I keep coming back is reimplementing the dedicated server for an old game like this. Even years ago when the games were popular, I often found the DS challenging to run.
I used to play so much Bunny Tracks. It was like digital parkour, using physics glitches for acrobatics through crazy obstacle courses. Such a neat game format, but I never saw anything like it (besides Portal, kinda.)