gistfile1.java

/**
 * Provides for nice formatting of tabular data.
 */
public class TextTable {
    private final List<Integer> maxWidths = Lists.newArrayList(); // widths for each column
    private final List<List<String>> rows = Lists.newArrayList();

    public TextTable addRow(Iterable<? extends Object> items) {
        return addRow(Lists.newArrayList(items));
    }

    public TextTable addRow(List<? extends Object> items) {
        final List<String> row = Lists.newArrayList();
        rows.add(row);

        for (int col=0; col < items.size(); col++) {
            final String item = items.get(col).toString();
            if (maxWidths.size() <= col) {
                maxWidths.add(item.length());
            } else {
                int maxWidth = maxWidths.get(col);
                if (maxWidth < item.length()) {
                    maxWidths.set(col, item.length());
                }
            }
            row.add(item);
        }
        return this;
    }

    @Override
    public String toString() {
        final StringBuilder builder = new StringBuilder();
        for (int rowIdx = 0; rowIdx < rows.size(); rowIdx++) {
            final List<String> row = rows.get(rowIdx);
            for (int colIdx = 0; colIdx < row.size(); colIdx++) {
                final String item = row.get(colIdx);
                builder.append(item);
                final int maxWidth = maxWidths.get(colIdx);
                for (int spacers=0; spacers < (maxWidth - item.length()); spacers ++) {
                    builder.append(" ");
                }
                builder.append(" ");
            }
            builder.append("\n");
        }
        return builder.toString();
    }
}

I hope you don't get angry on me giving opinions on your code (it's great, really), but just a small suggestion covering lines 38-43. :) As you are using Guava then why not use Strings.repeat(), isn't it?

final int maxWidth = maxWidths.get(colIdx);
builder.append(item)
         .append(Strings.repeat(" ",  (maxWidth - item.length())))
        .append(" ");

Also, lines 34-35, can be replaced by
for (final List row : rows) {