The best replacement for yslow is pagespeed.

The best alternatives to yslow are rated as following:
★★★★★  pagespeed
★★  fiddler
★  page speed

Your alternative for yslow is not listed? Suggest!
Speaking about yslow, one probably shouldn't forget about firebug.

Other search terms related to yslow and their ratings are:
★★★★★ firebug
You have in mind a companion for yslow and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of yslow in all years. Click on graph for more.
yslow
pagespeed
fiddler