Achieving Flat-End line buffers in QGIS?

If you use the v.buffer tool in the Grass Commands toolsets, you can make a flat buffer.

There there are two choice boxes If you have Make outside corners straight set to No, and Don't make caps at the ends of polylines as Yes, the result should be similar to the ArcMap straight edge buffer.

The following are the 3 combinations of the above two checkboxes, note the binary combination in the layer name (0 = no, 1 = yes).

You should probably also run a topology fixer/cleaner before, the Grass tool tends to return funny results if the topology isn't perfect

OpenJUMP may not suit you because all data must fit in memory but with 64 bit jre the limit is rather high. Most important, it can.