Are you a good photographer or illustrator? Don't hesitate to upload your files (Wikimedia Commons accepts only free content).
Help with categorising pages. There is a huge backlog at Category:Media needing categories.
There are a number of maintenance backlogs on commons requiring attention.
Wikimedia Commons is a multilingual project. You can help with translating pages into a language other than English. For example: Help translating the picture of the day's description.
Wikimedia Commons is a wiki which everyone can edit. Help identifying vandalism and detecting copyright violations.
Help developing user scripts & tools for the community, or tackle some related MediaWiki bugs.