Everything architecturally complex systems-related: upgrading, optimizing, creating new ones.
Bonus points for situations when I can put to use my knowledge of underlying systems and internals, and futher improve it!
- one-man projects, usually with analysis stages
- devops in team of 5 for student project
- deducing open source project behaviour from source code
- light profiling
- now trying a manager role for team of 10
- and lots of tinkering with packages and configs on various Linux distribs