What are the best things you do outside of work to advance your career? OSS? Side projects? Networking? Other?

Making things I release to the public. In one particular case I automated releasing 250 apps in a particular industry with a unified data source and got over 100k downloads from 2 weekends worth of development.

For me at least, side projects have been a huge part of reinvigorating my motivation and ultimately help me become better at what I do and avoid stagnating.

1. Podcasts (I particularly like listening to DevChat.tv shows)
2. Reading articles / e-books
3. Following tutorials online
4. Teaching others through articles, talks, or screencasts
5. Attending my local developer meetup

Attending random talks about the topics that I am working with or like to working with.
Online courses in science.

Whatever you enjoy doing that makes you learn new things or get to know new people.