SourceForge was for a long time, the de facto host for open source projects. Now it may have fallen a bit compared to the likes of GitHub and BitBucket, but it still hosts more than 400000 projects, 3.7 million registered users and has more than 4,800,000 downloads a day.
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Pro SourceForge offers a lot of helpful features for open source projects
Every project hosted on SourceForge can have discussion boards, and issue tracker, a tab for screenshots and something most code hosts don't have: a Shell access. Developers can also websites for their projects for free on SourceForge, as well as a Wiki for documentation.
The downside to this is that there is a lot of clutter in the beginning, and it may take a long time for beginners to check what they need and what they don't and where everything is.
Pro Easy for downloading projects
SourceForge makes it easy for users to download software hosted on it. It detects the user's platform and provides them with an appropriate version. They also have an extensive mirror network all over the world, which helps speed up downloads.
For developers, they offer download stats which are grouped by platform and by region.
Con Cluttered UI
Compared to other hosts, Sourceforge's UI feels messy and cluttered. Ads take up large portions of screen real estate, and feature creep has resulted in buttons and links everywhere that can sometimes make it difficult to find what you're looking for.
Con Downloads from SourceForge come with a closed-source installer which attempts to install third-party software
One of the main points of open source software is that users are getting a trustworthy product which they can trust, SourceForge has violated this trust in the past. Every project that is downloaded from SourceForge comes with it's (closed-source) installer which attempts to install third-party software in the computer. These third-party software, more often than not are adware or malware.
Dice, the new owners of SourceForge have chosen this as a way of monetisation and they strongly encourage developers to participate in this by giving them a cut of the profits.