My favorite one is Ion (complete, under active development). It is built on top of AndroidAsync (to use alone if you don't need Ion features), both by Koushik Dutta.
- Asynchronously download
- Easy to use Fluent API designed for Android
- HTTP POST/PUT
- Transparent usage of HTTP features and optimizations
- View received headers
- Grouping and cancellation of requests
- Download progress callbacks
- Supports file:/, http(s):/, and content:/ URIs
- Request level logging and profiling
- Support for proxy servers like Charles Proxy to do request analysis
- Based on NIO and AndroidAsync
- Ability to use self signed SSL certificates
