Like other major distributions, Ubuntu and Canonical have contributed to a number of projects that help their long-range plans. Upstart and Linaro, for example, have benefited hugely from Ubuntu and Canonical's interest. There are also countless Debian developers who have had more face-to-face interaction with their peers because of having their expenses paid by Canonical.
However, while such community contributions are rumored to be generous, they are not completely unusual among major FOSS projects, either. Instead, I would single out other features and policies as Ubuntu's major contributions to FOSS:
1. Simplified Installation
However, Ubuntu has done more to simplify and polish basic installation than any other distribution. Not only does the latest version of its installer include simple check boxes for various choices, such as whether to download updates while installing or encrypting your home folder, but the language of the installer has been refined to the point that it has eliminated much of the terror of installing an operating system.
Of course, the standard Ubuntu installer is not designed for users who want maximum control over the process. But its usefulness is indicated by the fact that it has been adapted by just about every distribution that new users might contemplate.
2. An Emphasis on Localization
Before Ubuntu, few distributions concerned themselves with language support. Most were Anglo-centric, and even switching keyboard layouts and fonts to support Western European languages was difficult. Ubuntu's first releases made graphic switching between multiple languages the norm.
Ubuntu's Launchpad was initially controversial because part of its code was proprietary. However, over the years, Launchpad has emerged as one of the great websites for hosting FOSS projects, along with SourceForge, Google Code, and The GNU Project.
However, because Launchpad focuses on Ubuntu development, much of the code hosted on it can be downloaded as a package, rather than needing manual compilation. Better yet, you can add a Launchpad site to your list of repositories and continue to update a piece of software you are following with a minimum of effort.
4. Regular Release Cycles
FOSS projects have always valued quality over scheduling. Recently, for example, Fedora delayed its latest scheduled release by two months, and just last year, openSUSE suspended its release schedule for several months so that the release team could re-evaluate its approach. The worst example is Debian, whose stability and software selection has often been paid for by gaps as long as several years between releases.
In this setting, Ubuntu's regular releases are almost unheard of. In contrast to the delays of months or even years in other distributions, Ubuntu is so regular that remembering the last release that was late by even a few days is impossible for the average user.