The one that stands out for me was my first major IT company. They gave a standard 25 day holiday allowance (about normal for UK), but they had an option right

The one that stands out for me was my first major IT company. They gave a standard 25 day holiday allowance (about normal for UK), but they had an option right on the intranet where you could buy or sell up to 5 days in either direction for a salary adjustment - so you could increase your holidays up to 30 days for a slightly lower monthly pay, or reduce it to 20 days and get paid a little more each month. It seemed like a great idea that I haven't seen anywhere else.