It's handy to have a virtually unlimited source of known weights. A few good options:

• US nickles are fairly close to 1N/$. Thus four nickles ($0.20) weight about 0.2N. We discovered this empirically but later confirmed it. Note that the official mass is 5g/nickle so at 1G (9.8m/s²) it's off by 2%. But for most of what we do there are multiple bigger sources of error.
• Water weighs (masses?) about 1g/cm³ (why not say ml?). This still has the problem that 1G isn't quite 10m/s² and also depends on temperature and salt content, but oh well.