Here are some interesting projects in which I've been able to contribute, sometimes very lightly, sometimes thoroughfully:
GNOME is one of two main desktop environments for GNU/Linux (the other one being KDE). As with all other Free Software project, any contributor with required skills is welcome.
Ubuntu is a GNU/Linux distribution based on Debian. It's very popular. Translation of specific aspects of this distro is done through Launchpad, the Web-based collaborative system developed by Canonical, the founder of Ubuntu.
Django is a high-level Python Web framework that encourages rapid development and clean, pragmatic design.
RapidSMS is a Web application based on the Django framework and which serves to centralize SMS-received data. Possible usages are numerous and you can browse some noteworthy case-studies in humanitarian projects in Africa, where the GSM coverage is notably wider than Internet access.