IP Georeferencing is a pretty cool toolbox item on today's web. Simply put, its the process of taking an IP address and converting it to some geographical (city, lat, long, whatever) information.
It gets commonly used for security filters, log analysis, ad targeting, community building, etc..
Maxmind deserves a shout out for making available databases to do this. There is nothing inherent about an IP number that gives you this information, so they just ned to build out of band databases to get the job done. They have two copies of the databases, one for free and one a bit more accurate that is priced very reasonably.
The databases come with a number of different libraries for using them: C, Java, PHP, etc.. The libraries are released under the LGPL.
Recently I was doing a project that needed to lookup scads and scads of addresses, so I put a little muscle into improving the lookup routines in the C code.
I'm happy to say I was able to improve things anywhere from 2x to 8x in terms of overall lookups, depending on what exactly was being lookedup. There were a bunch of changes, but the primary one was the addition of a variable length radix lookup mechanism that changed the average number of comparisons from 27 to 4 - not rocket science, but the right tool for the job.
I'm even more happy to say I sent those back as LGPL contributions of my own. The code is on my open source contribution web page.