For me, the tool doesn't have to be open source. If I'm looking for a piece of software to do something, then I will consider commercial and open source applications. But the main things I look for on a website are:
1. Screenshots
Perhaps the most important thing for me is to see how a program looks before I know everything it does. If the user interface is clear and helps me accomplish the task in hand, then it's likely that I'll give the program a try. If a website has no screenshots then I'm a lot less likely to download, given that there is a lot of malware on the Internet.
If it's a game, then presentation is just as important, if not more so. If a game looks good by the screenshots, it'll almost certainly be downloaded.
2. Feature list
Even if it's only a couple of paragraphs on the main page, I like to see an application's main features. Not an exhaustive list of what the developer thought was cool, but half a dozen or so key selling points.
3. Recent news entries
One of the key things is to have a website that is updated regularly. Software invariably has bugs and or missing features, and it's discouraging to see a website that's 3 years old and has only had a handful of minor fixes. Even if nothing has been done to the software, it is nice to have developer reports of why that is, rather than the occasional update every year.
4. Support Forums
An active community is a great advertisement for any piece of software. If there's a large userbase behind your project then it's easier to see the feedback, bugs, and features that will be added in future releases.
5. Documentation
One of the elements that is missing from many projects is documentation. Yes, you know what your program does, but do your users? No. When I've downloaded the program I need to know how to use it! Documentation is no good when it's out of date, so it needs to be updated regularly.
OpenTTD.org is a great example of how it's done. There's a large, active community, documentation (wiki), screenshots, and a news page. You can even donate to them if you like (I don't consider this an essential part of a website - but it is an important one nonetheless). Also, it's a great game that's come a long way, and new releases are regular.