If you are new to lifting straps, this sequence of photos should guide you on how to hool them up to assist the grip of a dumbbell.
You can buy them (for example here) but you can also make them - which can be fun. My own sandbag (seen in the photo) is a pretty crude effort, knocked up by a builder friend of mine using tape and canvas. It's 28kg. It leaks and is not robust enought to withstand throwing. I use it mainly for lifting and pressing.
Here's a good tutorial on making sandbags from Ross Enamait.
Weighted VestThis bad boy has served me well. There are many varieties of weighted vest, but this Reebok version, the one I own, allows you to remove half pound inserts from pouches to vary the weight between 1 and 20lb. You can also buy additional inserts to get up to 40lb.
You can use it to make bodyweight exercises harder - press ups, burpees, pistols, ab rollouts - anything, in fact, where the range of movement will not be affected. For pull ups and chin ups I would instead recommend the weighted approaches detailed in those posts.
It's also good for making running harder - I like to use it on short hill runs.
From workout diary: weighted vest sessions