Using open source software to merge large shapefiles?

From the GDAL manual:

A merge of two shapefiles 'file1.shp' and 'file2.shp' into a new file 'file_merged.shp' is performed like this:

% ogr2ogr file_merged.shp file1.shp
% ogr2ogr -update -append file_merged.shp file2.shp -nln file_merged

The second command is opening file_merged.shp in update mode, and trying to find existing layers and append the features being copied.

The -nln option sets the name of the layer to be copied to.

http://www.gdal.org/ogr/drv_shapefile.html

Try converting the shapefiles to a file geodatabase and then merge them in ArcGIS.

You may even be able to convert them back to shape after the merge (in case you're trying to maintain compatibility with other software that relies on shapefiles).

You could try Geomerge. I'm not sure how many records it can handle, but so long as the table structure is the same for all files I have not had any problems with it.

The MapBrowser available on the same website is also an excellent little tool for quick viewing of shapefiles without having to open up a GIS package.