In many ways! Here are some:
Using Trisquel, and sharing it with your friends and family
Reporting issues and bugs that you find, and suggesting improvements
Helping in the forums
Writing and translating documentation
Making a donation
Becoming an associate member
Buying some T-shirts and other gear at our store
Translating applications
If you have experience developing software, check the contributing guidelines
If you would like to volunteer, write us.