Onset putters are a little hard to come by, but can make a big difference to your putting. The definition of onset is that the shaft attaches into the head in such a way that the face of the putter is "ahead" (i.e. closer to the target of your putt) than the leading edge of the shaft.
There are two major why you might want to choose an onset design:
          1. You can see the whole of the top edge of the putter blade without your vision being obscured by the shaft.
          2. Your aiming may improve by using an onset design. Although this is not always the case, the majority of people aim further right with an onset putter and further left with an offset putter.
Unless you invest in a fitting with a well trained putter fitter who can measure how accurately you aim a variety of putters with offset and onset, your best option is going to be to try out the putters you like from short range (say 2-3 feet) and see how consistently you can get the ball going right into the middle of the hole.
If you're sneaking too many in at the sides of the hole from this range, it's likely that you're not aiming the putter very well. If that's the case from 2-3 feet you're almost never going to succeed from 10 or 20 feet.
Click on any of the links below for an unbiased review and to find out where you can buy them at the best price.
Odyssey Backstryke
One of the most revolutionary designs in putting. Love it or hate the look, the results are good
Ping
Ping 1A
The first putter Ping ever made. A collectors' piece now. Not great to use, and very loud!
Ping Scottsdale Pickemup
This cross between the Batman sign & a Star Wars fighter has surprisingly good feel
Seemore
Seemore FGP Original
A revolutionary putter which has now won 2 major championships thanks to its combination of roll on fast greens and patented riflescope aiming design. Well worth considering!
Titleist
Titleist Bullseye Flange
An extremely pretty putter. A favourite with some of the world's greats 50 years ago and excellent on fast greens. Unforgiving when compared with modern putters