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

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

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.

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

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