Matteo Pomponi has created a new web-based Slackware Packages Tracker which can be used to track/search Slackware packages resides in official, SlackBuilds, or universe repository (slacky.eu).
So far, the application are only being set up to search for Slackware 13.37 and -Current (both 32 and 64 bit), but it can be easily enhanced to support more version in the future because all Slackware packages are highly structured. As for third party repository, the Tracker also displayed the required dependency, thanks to the slack-required file that tracks the dependency packages needed.
Although it's still marked as testing, but in overall, all of the functions are working nicely, but beware when you tried to search a package name that are so general, for example, "lib". Try to filter the repository and architecture first so that you will not have to wait for too long.