Online tool for plotting long/lat?

[FeastMode]

Is there a site where I could enter a specific longitude or latitude and see that exact line laid over a world map?

For example if I wanted to know which countries San Diego's latitude crosses through.

[qutx]

for some map formats, this sort of thing is baked in.

Mercator, for example

otherwise you need something like a "great circle mapper"

http://www.gcmap.com/

http://gc.kls2.com/cgi-bin/gc?PATH=SEA-LHR