On the research list as it seems a lot wider than HTTP Fiddler:
Paros is a valuable testing tool for your security and vulnerability testing. Paros can be used to spider/crawl your entire site, and then execute canned vulnerability scanner tests. But Paros goes beyond that, it comes with a built in utility that can proxy traffic. This Paros Proxy utility can be used to tamper or manipulate any http or https traffic on the fly. This makes some of the more interesting security types of testing. It will help you isolate potential area’s of security concern and then manual attempt to perform the type of testing you desire.
Paros also comes with a built in Session ID analyzer. It will display a graph of all the types of Session ID’s it has been presented with using a multiple threaded session initiator. You then can determine if the graph appears random enough for the Session ID. It is a pretty unique and interesting tool to use. Although typically most developers will rely upon another technology tomcat, apache, or some other application to generate Session ID’s. This is not always the case and as such a Session ID analysis should be performed. Sometimes the Session ID will not be randomized enough and the hash used to create the Session ID is easily predictable.
Paros also comes with a built in Fuzzer. You will need to generate your own Fuzzer library to use the Fuzzer, but it will perform all the fuzzing for you.