Debian has its own official repositories, which have different distributions:
- Stable: currently lenny
- Testing: currently squeeze
- Unstable: will always be sid
Besides, there are two other official repositories which provide security updates, and volatile, which provides packages that move faster than the stable release (I recomend you to read that link). Volatile may be the one you are looking for if you whish to maitain sanity, without going to testing or unstable (which I do not recomend for production). The sources you should use are (/etc/apt/sources.list):
deb http://volatile.debian.org/debian-volatile lenny/volatile main contrib non-free
deb http://volatile.debian.org/debian-volatile lenny/volatile-sloppy main contrib non-free
Be sure to read the link before adding those sources. Other ways are to use backports (now an official service at debian) or provided deb packages by official software maintainers.