s2map is a simple geo visualizer
source code on gitbhub

I find I have one or more lat/lngs, and I want to put them on a map and not worry about formatting, writing a file, etc, so I just paste it into s2map.com I'm curious if other people find this useful or have feature suggestions.

oh, and just for kicks alt/ctrl/meta+click pops up a balloon with lat/lng info

Additionally, s2map demonstrates the power of google's s2 (spherical geometry library) available for C++ (which doesn't compile publicly, the version in my github repo does) and java (in extensive use at foursquare). We use it as a very robust geohashing library with a number of great properties (skip-level indexing, prefix parents). Read the code, it's beautiful and well documented and a pretty good intro to the concepts. Presentaion on the algorithms and features.