I have used YSlow in the past & was curious to know how it differs from Page Speed. There are a few articles that talk about the differences but the Page Speed FAQ sums up it up well -
YSlow and Page Speed use some common "rules" that are based on industry best practices. There are, however, some rules, such as deferring JavaScript until after the page has loaded, that Page Speed implements exclusively. In addition, Page Speed provides you with an activity panel that allows you to do a "before and after" comparison when considering the implementation of Page Speed's suggestions. Finally, Page Speed is an open-source tool that is open to community contributions.
YSlow & Page Speed can be used together but using them both may not be a good idea as it will make Firefox considerably slower. A Stack Overflow user has suggested using a seperate Firefox profile to handle that issue.
Also see:
neXpert - YSlow clone for Fiddler
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