This quote suggests that philanthropy, or the act of giving to others, is one of the few virtues that people universally value and appreciate. Thoreau is acknowledging a shared human respect for generosity and selflessness, implying that other virtues may not be as universally recognized or appreciated.

The quote also subtly hints at a critique of society. By stating that philanthropy is "almost the only virtue" appreciated by mankind, Thoreau is suggesting that there are numerous other virtues that society does not value enough. This could be virtues like humility, patience, or honesty. It’s an invitation to reflect on what we, as a society, value and why.

Applying this quote to today’s world, it can be seen in the way society often celebrates philanthropic acts, especially by high-profile individuals or corporations. We see headlines about billionaires donating large sums of money, and while that is commendable, it’s important to remember that philanthropy isn’t the only measure of virtue.

In terms of personal development, this quote can serve as a reminder to strive for a well-rounded character. While being generous and helping others is important, it’s also crucial to cultivate other virtues such as integrity, kindness, and perseverance. It can also serve as a prompt to question societal norms and values, and to form one’s own understanding of what it means to be virtuous.