PROJ.4

• Use proj tool directly, at the commandline

 $ proj -lp    # projections
 $ proj -le    # ellipsoids
 $ proj -ld    # datums

• Use R (rgdal package) to do the same thing

 # R code:
 library(rgdal)
 projInfo("proj")    # projections
 projInfo("ellps")   # ellipsoids
 projInfo("datum")   # datums

The EPSG code can be a convenient way to specify a projection in QGIS, ogr2ogr, proj etc. The saves the hassle of inputting the full PROJ.4 definition string, which can be an error prone step. However, be aware that not all projections are associated with an EPSG code.

• Use the web: Online spatial reference search

• Use R (rgdal package)

 # R code:
 library(rgdal)
 EPSG <- make_EPSG()
 # As an example, search for Hawaii state projections
 EPSG[[grep("hawaii",|EPSG$note, ignore.case=TRUE), 1:2]]
 # Get PROJ.4 information for a particular EPSG code
 subset(EPSG, code==2784)

• PROJ.4 wiki page documenting the general PROJ.4 parameters available for defining (most) coordinate systems.
• Flex Projector is a free, cross-platform Java application that provides a graphical interface for manipulating map projections. Designed primarily to enable users to create arbitrary new projections for custom map visualizations, this tool can be quite educational for those who simply want to get a sense of how and why projections differ.