What are your criteria for choosing a framework or library?

- It must have no run-time license fee: my employers (ISVs) can pay a per-developer license, but usually not a per-customer or per-end-user license.

- I must be able to support it (which means my having source code): in case the vendor goes out of business, won't fix a bug, or won't add new functionality which we want in our next release.

I want something that:

- works (you really can't over-emphasize this)
- is well written (it probably needs to be OSS so I can look at this)
- is well maintained - I want to see updates on a regular basis
- has a clean interface
- I can fix if it breaks (again, OSS is a win)
- doesn't lock me into a single vendor

I like tvanfosson's answer above.

Having poor documentation or a bad looking home page is a turn off but doesn't push me away as fast a buggy code or a poor API.

As for using third party libs where the language already has support, I use the jodatime library rather than Java's built in date handling (which I think most people agree sux.) I was thrilled to hear that they are redoing date handling more like Joda's implementation.

If there is an open source option that works I will always choose that, there is no better support then having the source code and being able to make changes to suit your needs.
If there is no api documentation I don't waste my time with it.

Edit: @[Josh Brown][1] comment

Yes I have actually had to make changes to open source libraries I have used. I used a library for th first version of a project, then the requirements for the next version required me to add a new feature. It was easier to modify the library myself then to find a new that supported the new feature and then modify my code to work with the new library.


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In order of importance:

1. It is easy to understand and/or well documented (tutorials are the best).
2. I can use it the same at work and at home (so open-source is helpful, but copyleft not)
3. It has recent activity.
4. It is stable. Some indications for it are a version >= 1.0.
5. The project tries to keep compatibility to earlier version. Nothing is more annoying than getting the new version of the framework for the new feature that would be handy for your app - and then recognizing that your app no longer works with the framework.

Some projects that match these criteria:

* [Java][1]
* The [Apache Commons libraries][2] I used so far (IO, Logging, Math)
* [JTS Topology Suite][3]

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