A clean, lightweight alternative to Python's twisted?

Twisted is complex, you're right about that. Twisted is not bloated.

If you take a look here: http://twistedmatrix.com/trac/browser/trunk/twisted you'll find an organized, comprehensive, and very well tested suite of many protocols of the internet, as well as helper code to write and deploy very sophisticated network applications. I wouldn't confuse bloat with comprehensiveness.

It's well known that the Twisted documentation isn't the most user-friendly from first glance, and I believe this turns away an unfortunate number of people. But Twisted is amazing (IMHO) if you put in the time. I did and it proved to be worth it, and I'd recommend to others to try the same.

gevent is eventlet cleaned up.

API-wise it follows the same conventions as the standard library (in particular, threading and multiprocessing modules) where it makes sense. So you have familiar things like Queue and Event to work with.

It only supports libevent (update: libev since 1.0) as reactor implementation but takes full advantage of it, featuring a fast WSGI server based on libevent-http and resolving DNS queries through libevent-dns as opposed to using a thread pool like most other libraries do. (update: since 1.0 c-ares is used to make async DNS queries; threadpool is also an option.)

Like eventlet, it makes the callbacks and Deferreds unnecessary by using greenlets.

Check out the examples: concurrent download of multiple urls, long polling webchat.

I liked the concurrence Python module which relies on either Stackless Python microthreads or Greenlets for light-weight threading. All blocking network I/O is transparently made asynchronous through a single libevent loop, so it should be nearly as efficient as an real asynchronous server.

I suppose it's similar to Eventlet in this way.

The downside is that its API is quite different from Python's sockets/threading modules; you need to rewrite a fair bit of your application (or write a compatibility shim layer)

Edit: It seems that there's also cogen, which is similar, but uses Python 2.5's enhanced generators for its coroutines, instead of Greenlets. This makes it more portable than concurrence and other alternatives. Network I/O is done directly with epoll/kqueue/iocp.